Matthias ; Thürer

Three Decades of Workload Control Research: A Systematic Review of the Literature

Keywords: PRODUCTION PLANNING, ORDER Workload Control, Literature Review Workload Control (WLC), Order Review and Release (ORR), Production Planning and Control (PPC), Systematic Literature Review

The Workload Control (WLC) concept has received much attention in the past three decades; however, a comprehensive literature review has not been presented. In response, this paper provides a systematic review of the conceptual, analytical, empirical and simulation-based WLC literature. It explores the evolution of WLC research, determines the current state-of-the-art and identifies key areas for further study. The research finds that the field has evolved substantially. Early research focused on theoretical development and experimental testing of order release strategies; order release was then integrated with other planning stages, e.g., the customer enquiry stage, making the concept more suitable for customised manufacturing and leading to a comprehensive concept which combines input and output control effectively; recent attention has focused on implementing the resulting concept in practice and refining theory. While WLC is well placed to meet the needs of producers of customised products, future research should include: conducting further action research into how WLC can be effectively implemented in practice; studying human factors that affect WLC; and, feeding back empirical findings to simulation-based WLC research to improve the applicability of WLC theory to real-life job shops.

Introduction

The Workload Control (WLC) concept was developed to overcome the "lead time syndrome" [START_REF] Mather | Priority fixation versus throughput planning[END_REF]. Job entry is decoupled from release; orders are held back in a pre-shop pool and input to the shop floor is regulated in accordance with workload limits or norms. The objective is to maintain WIP at an optimal level and keep queue lengths in front of work centres short. The output rate is manipulated by adjusting capacity and it has been shown that the two control mechanisms complement each other, i.e., input should be regulated in accordance with the output rate [START_REF] Kingsman | The relative contributions of input and output controls on the performance of a workload control system in Make-To-Order companies[END_REF]. WLC stabilises WIP and lead times, enabling production and inventory costs to be reduced and both competitive prices and reliable Due Dates (DDs) to be quoted. It is considered a leading Production Planning and Control (PPC) solution for Make-To-Order (MTO) companies, where pricing and DDs have to be determined for each job and are crucial order winning factors [START_REF] Kingsman | The relative contributions of input and output controls on the performance of a workload control system in Make-To-Order companies[END_REF][START_REF] Stevenson | A review of production planning and control: The applicability of key concepts to the make to order industry[END_REF], and particularly appropriate for Small and Medium sized Enterprises (SMEs) with limited financial resources [START_REF] Stevenson | A review of production planning and control: The applicability of key concepts to the make to order industry[END_REF][START_REF] Land | Production planning and control in SMEs: Time for change[END_REF].

WLC research has been conducted throughout the last three decades; however, it was not until Zäpfel & Missbauer (1993b) that the term "WLC" was first used to refer to a group of PPC methods. The authors referred to "PPC system[s] including WLC", grouping together three streams of research which seek to control workloads: Order Review and Release (ORR) methods, largely developed in North America (e.g., [START_REF] Melnyk | Order review/release: research issues and perspectives[END_REF][START_REF] Melnyk | Load smoothing by the planning and order review/release systems: A simulation experiment[END_REF][START_REF] Ahmed | Due date assignment, job order release and sequencing interaction in job shop scheduling[END_REF]; workload controlling methods building on input/output control (I/OC, from [START_REF] Plossl | Capacity planning and control[END_REF], largely developed in the UK at Lancaster University (e.g., [START_REF] Tatsiopoulos | Lead time management[END_REF]Hendry & Kingsman 1991a;Hendry & Kingsman, 1993);and, Load Oriented Manufacturing Control (LOMC), largely developed at Hanover University in Germany (e.g., [START_REF] Bechte | Theory and practise of load-oriented manufacturing control[END_REF][START_REF] Wiendahl | Application of load-oriented manufacturing control in industry[END_REF][START_REF] Bechte | Load-oriented manufacturing control just-in-time production for job shops[END_REF]. More recently, Land & Gaalman (1996a) reviewed order release rules that seek to control workloads and integrated these into a comprehensive PPC system, hereafter referred to as "ORR WLC". Finally, [START_REF] Hendry | The effect of workload control (WLC) on performance in make-toorder companies[END_REF] consolidated the four streams of research (i.e., ORR, I/OC, LOMC, and ORR WLC) under the 'umbrella term' of "WLC", designating it a new group of PPC concepts to control queues in job shops. Nowadays, all four of the concepts referred to above are generally accepted as being part of WLC research.

Elements of WLC research have been referred to in several reviews of a range of PPC concepts (e.g., Hendry & Kingsman, 1989;Zäpfel & Missbauer, 1993b;[START_REF] Stevenson | A review of production planning and control: The applicability of key concepts to the make to order industry[END_REF]; however, these studies are too broad to go into sufficient depth on each concept. Other studies have attempted to provide an overview of WLC research but have tended to focus on describing the various ORR 3 mechanisms (e.g., [START_REF] Melnyk | Order review/release and its impact on the shop floor[END_REF][START_REF] Wisner | A review of the order release policy research[END_REF][START_REF] Bergamaschi | Order review and release strategies in a job shop environment: A review and a classification[END_REF] or comparing them through simulation (e.g., [START_REF] Philipoom | An evaluation of capacity sensitive order review and release procedures in job shops[END_REF][START_REF] Sabuncuoglu | Analysis of order review/release problems in production systems[END_REF]) and hence do not incorporate all PPC stages within the scope of WLC. Moreover, few recent reviews of the PPC literature have been presented -most of the aforementioned studies were published in the 1980s and 1990s, thus recent developments (e.g., since 2000) have not been considered. It follows that a comprehensive contemporary review is required which focuses only on WLC and covers all of the PPC stages within its scope.

In response, this paper provides a systematic review of the conceptual, analytical, empirical and simulation-based WLC literature published between 1980 and 2009, with a particular focus on the last decade. It consolidates the WLC literature to date, explores the evolution of WLC research, and identifies outstanding gaps for future research. Research relating to all of the concepts above (ORR, I/OC, etc) are included in the review providing that the objective is to control the workload directly.

On the other hand, Constant Work-In-Process (ConWIP) is not included in the review as it only controls workload indirectly (based on the number of jobs in the system).

The remainder of the paper is organised as follows. Section 2 outlines the systematic method behind the review -including how the literature was categorised -before Section 3 briefly defines WLC. The literature review is presented in Section 4 -this includes identifying key research gapsbefore final conclusions follow in Section 5.

Methodology

This review began by considering the following research questions (RQ1 & RQ2): RQ1: What have been the main contributions to the field of WLC? And has the focus of WLC research shifted over the past three decades? In other words, how is the field evolving? RQ2: What are the most important future research directions in the field of WLC? In other words, how should the field of WLC evolve in the future?

A WLC database was built for the systematic review through a four-stage process. Firstly, papers published in international Business & Management journals were analysed (www.b-on.pt) and all appearing potentially relevant to WLC (including ORR, I/OC, etc) were shortlisted. Secondly, the shortlisted articles were carefully read to assess the true relevance; if relevant, the papers passed into a preliminary database. Thirdly, papers cited in the articles identified during the second stage were also read carefully to determine relevance to WLC; this ensured that relevant articles not identified during the first step were not overlooked. Fourthly, all articles in the database related with

Categorisation of Literature

In his review of order release policies, [START_REF] Wisner | A review of the order release policy research[END_REF] divided research into: descriptive, analytical and simulation-based research. Descriptive research contained general discussion papers, case study research and survey research. Only two empirical studies were included [START_REF] Igel | Experience with job shop planning[END_REF][START_REF] Bechte | Controlling manufacturing lead time and work-in-process inventory by means of load-oriented order release[END_REF] but, in this review, there are a further nine. The above categorisation is therefore adapted to: conceptual, analytical, empirical, and simulation-based (conceptual corresponds to the descriptive category from [START_REF] Wisner | A review of the order release policy research[END_REF] excluding empirical research). Almost all articles could be categorised as conceptual but only those which do not fall under one of the other categories are included.

Workload Control (WLC): An Introduction

Many WLC methods are described in the literature; the unifying theme is use of a pre-shop pool and order release mechanism. All regulate release by considering the current load (e.g., at each work centre), workload limits and job characteristics (e.g., DD and workload). WLC methods emerging from the classical ORR concept and viewing WLC as the interface between the planning system and the shop floor have three control levels: job entry; job release; and, priority dispatching. Land & Gaalman (1996a) combined these into a comprehensive hierarchical concept referred to here as the ORR WLC concept. The WLC methods based on I/OC, largely developed at Lancaster University and hereafter referred to as the LUMS Approach, added the customer enquiry stage to create a four-tiered system. Figure 1 illustrates the control levels of the ORR WLC concept and the LUMS Approach; each control level is briefly described below.

[Take in Figure 1]

Customer Enquiry and Job Entry Stages

Much research was conducted in the 1980s into setting adequate DDs (e.g. Bertrand, 1983b;[START_REF] Ragatz | A framework for the study of due date management in job shops[END_REF] and throughout the 1990s many ORR researchers sought to find the best fit between DD assignment, order release and dispatching rules (e.g., [START_REF] Ahmed | Due date assignment, job order release and sequencing interaction in job shop scheduling[END_REF]. A key finding was that DD rules which consider shop and job information perform better than those which do not [START_REF] Ragatz | A framework for the study of due date management in job shops[END_REF].

The customer enquiry stage, as included in the LUMS Approach, takes place between a customer making a request for quotation and an order being accepted/rejected [START_REF] Kingsman | Responding to customer enquiries in make-to-order companies: Problems and solutions[END_REF]. It includes determining whether to bid for an order and, if so, what the DD and price should be. The LUMS Approach considers both shop and job information and incorporates a proportion of the workload of unconfirmed jobs in the total workload of the shop based on the probability of winning a tender [START_REF] Kingsman | Strike rat matrices for integrating marketing and production during the tendering process in Make-to-Order subcontractors[END_REF]. Much recent research has focused on this stage; for example, [START_REF] Kingsman | Modelling input-output workload control for dynamic capacity planning in production planning systems[END_REF] proposed an analytical model for dynamic capacity planning at the customer enquiry stage and [START_REF] Kingsman | The relative contributions of input and output controls on the performance of a workload control system in Make-To-Order companies[END_REF] highlighted the importance of input/output control at this stage. Order entry begins with order acceptance/rejection and includes pre-production preparations for confirmed orders (e.g., checking material availability).

Job Release Stage

Two order release methods have dominated WLC research: the probabilistic and aggregate approaches. The release procedure is similar in both [START_REF] Land | The performance of workload control concepts in job shops: Improving the release method[END_REF]: jobs are held in a pre-shop pool where they are considered for release, e.g., according to shortest slack, latest release date, or first-come-first-served. The load of a job is compared with the current load and limits of work centres and, if one or more limits would be exceeded by releasing the job, it is retained in the pool until the next release date. If the limits are not exceeded, the job is released and its load contributes to that of the work centres. The norms can be upper bound, lower bound, or upper and lower bound and either rigid or flexible.

The main difference between the approaches is how they treat the indirect load, i.e., how the workload of a job that is still upstream of a given work centre is handled:

• The probabilistic approach estimates the input from jobs upstream to the direct load of a work centre using a depreciation factor based on historical data. When a job is released, its processing time partly contributes to the input estimation; the contribution increases as the job progresses downstream. The whole of the direct load and the estimated input is indicated as the converted load [START_REF] Bechte | Load-oriented manufacturing control just-in-time production for job shops[END_REF][START_REF] Wiendahl | Load oriented manufacturing control[END_REF]. The approach was introduced by [START_REF] Bechte | Steuerung der Durchlaufzeit durch belastungsorientierte Auftragsfreigabe bei Werkstattfertigung[END_REF]1982) and known as Load Oriented Order Release (LOOR); LOOR formed the basis of the Load Oriented Manufacturing Control (LOMC) concept [START_REF] Bechte | Theory and practise of load-oriented manufacturing control[END_REF][START_REF] Bechte | Load-oriented manufacturing control just-in-time production for job shops[END_REF][START_REF] Wiendahl | Load oriented manufacturing control[END_REF].

• The classical aggregate load approach, introduced by [START_REF] Bertrand | Production control and information systems for componentmanufacturing shops[END_REF] and [START_REF] Tatsiopoulos | A microcomputer-based interactive system for managing production and marketing in small component manufacturing firms using a hierarchical backlog control and lead time management technology[END_REF], does not consider the position of a work centre in the routing of a job. The direct and indirect workloads of a resource are simply aggregated together. [START_REF] Tatsiopoulos | A microcomputer-based interactive system for managing production and marketing in small component manufacturing firms using a hierarchical backlog control and lead time management technology[END_REF] developed a variant of this called the extended approach which controls the shop load rather than the load of each individual work centre to overcome problems caused by a lack of feedback from [START_REF] Oosterman | The influence of shop characteristics on workload control[END_REF]. Land & Gaalman (1996b) proposed a further extension, the corrected aggregate load approach, which divides the load by the position of a work centre in the routing of a job thereby converting the load (like the probabilistic approach) but without requiring statistical data. This approach arguably performs the best of the above, especially if a dominant flow exists [START_REF] Oosterman | The influence of shop characteristics on workload control[END_REF].

Dispatching Stage

Much research into dispatching took place in the 1980s and 1990s, with many authors underlining the importance of an appropriate dispatching rule (e.g., [START_REF] Melnyk | Order review/release: research issues and perspectives[END_REF][START_REF] Ahmed | Due date assignment, job order release and sequencing interaction in job shop scheduling[END_REF]. However, the choice of dispatching rule becomes less significant when combined with other control levels. For example, [START_REF] Ragatz | An evaluation of order release mechanisms in a job shop environment[END_REF] stated that order release rules reduce differences between dispatching rules as the number of shop floor jobs is reduced. Most contemporary WLC research applies only simple dispatching rules; however, there are exceptions.

For example, [START_REF] Stevenson | Refining a workload control (WLC) concept: A case study[END_REF] applied a special dispatching policy for priority jobs.

Literature Review and Future Research Directions

This section is structured as follows. Firstly, for each research category (conceptual, analytical empirical and simulation-based research; see sections 4.1 to 4.4, respectively), key WLC research from the 1980s and 1990s is reviewed in order to explore how the field has evolved and build the backdrop for the analysis of the literature since 2000. Secondly, recent literature published since 2000 is reviewed in light of the research from the 1980s and 1990s in order to identify changes in the focus of research and outstanding research gaps. Thirdly, future research directions for each category are outlined. 4.1.1 Conceptual Research (1980-1999) Four types of conceptual research were conducted in the 1980s and 1990s: (1) the categorisation of WLC; (2) reviewing different PPC concepts and WLC; (3) developing the theory of the LUMS Approach; and, (4) developing the theory of LOMC. The first group mainly consists of [START_REF] Wisner | A review of the order release policy research[END_REF] and [START_REF] Bergamaschi | Order review and release strategies in a job shop environment: A review and a classification[END_REF] who categorised order release policies. The second group consists of the reviews by Hendry & Kingsman (1989), Zäpfel & Missbauer (1993b) and Land & Gaalman (1996a). For example, Hendry & Kingsman (1989) [START_REF] Tatsiopoulos | Lead time management[END_REF] and [START_REF] Kingsman | A structural methodology for managing manufacturing lead times in make-to-order companies[END_REF] outlined the concept before it was further developed, for example, by Hendry & Kingsman (1991a) and Hendry & Kingsman (1991b). Hendry & Kingsman (1993) presented theory for controlling the Total and Planned Backlog Lengths (TBL and PBL) simultaneously; [START_REF] Kingsman | Integrating marketing and production planning in make-to-order companies[END_REF] outlined the importance of integrating production and sales, introducing the use of the strike rate; and, [START_REF] Kingsman | Responding to customer enquiries in make-to-order companies: Problems and solutions[END_REF] presented an approach for determining prices and DDs. Researchers in the fourth group developed the LOMC concept. These papers (e.g., [START_REF] Bechte | Theory and practise of load-oriented manufacturing control[END_REF][START_REF] Bechte | Load-oriented manufacturing control just-in-time production for job shops[END_REF] made important conceptual contributions but theory was typically developed through empirical insight and hence the papers are also included in Section 4.3 (empirical research).

Conceptual Research

At the end of the 1990s, two decades of conceptual research had contributed to the development of two mature WLC systems: the LUMS Approach, a comprehensive PPC system; and, LOMC, a widely implemented solution for integrating a planning system with the shop floor. Research (2000Research ( -2009) ) Four conceptual research directions were identified in the 1980s and 1990s. Research continued in all four areas but with most attention on Group 3: developing the theory of the LUMS Approach.

Conceptual

The only contribution to Group 1 was Henrich et al. (2004a) who introduced a framework for analysing the characteristics of a company and assessing WLC applicability. This is an important contribution but more research is needed to delimit WLC from other PPC concepts (e.g. ConWIP) especially if it is to be compared with these concepts, as by researchers in Group 2. The main contribution to Group 2 was [START_REF] Stevenson | A review of production planning and control: The applicability of key concepts to the make to order industry[END_REF] who assessed the applicability of several PPC concepts to different shop characteristics. As in previous reviews, WLC was found to be one of the best solutions for MTO companies. The other contribution was made by [START_REF] Fowler | Workload control in the semiconductor industry[END_REF] who assessed the applicability of different PPC systems to the semi-conductor industry considering Starvation Avoidance (SA), developed especially for wafer fabrication by [START_REF] Glassey | Closed-loop job release control for VLSI circuit manufacturing[END_REF]. The remainder of this subsection focuses on groups 3 and 4 where the emphasis has shifted from theory development to theory refinement.

Since 2000, the LUMS Approach has been refined according to theoretical advances and contextual changes [START_REF] Stevenson | Aggregate load oriented workload control: A review and a reclassification of a key approach[END_REF] and in response to issues encountered whilst implementing WLC, including human factors (e.g., [START_REF] Silva | MAPP -A web-based decision support system for the mould industry[END_REF][START_REF] Stevenson | Theoretical development of a workload control methodology: Evidence from two case studies[END_REF][START_REF] Hendry | Investigating implementation issues for workload control (WLC): A comparative case study analysis[END_REF][START_REF] Huang | Organizational embedding of a workload control concept: An action research project[END_REF]. Refinements in response to theoretical advances included removing the lower bounding of workloads introduced by Hendry & Kingsman (1991a) following the simulation results of [START_REF] Cigolini | An experimental investigation on workload limiting methods with ORR policies in a job shop environment[END_REF]; refinements in response to contextual changes included controlling daily rather than weekly total and planned workload lengths to cope with shorter lead time demands. Implementation issues encountered included a lack of familiarity in practice with WLC, hindering progress during the early stages of a project [START_REF] Silva | MAPP -A web-based decision support system for the mould industry[END_REF][START_REF] Stevenson | Theoretical development of a workload control methodology: Evidence from two case studies[END_REF][START_REF] Hendry | Investigating implementation issues for workload control (WLC): A comparative case study analysis[END_REF]. In response, [START_REF] Stevenson | The development and application of an interactive end-user training tool: part of an implementation strategy for workload control[END_REF] developed an interactive end-user training tool which coupled a DSS based on the LUMS Approach with a simulated shop floor and demonstrated its positive impact in practice. In other cases, refinements were made without validation. For example, [START_REF] Stevenson | Refining a workload control (WLC) concept: A case study[END_REF] introduced the option of releasing part of a job from the pool but did not evaluate the impact on overall release performance while [START_REF] Stevenson | Theoretical development of a workload control methodology: Evidence from two case studies[END_REF] compared refinements made during two implementations of the LUMS Approach conducted independently but in parallel and found that few refinements were valid for both cases.

A need for web-functionality within a WLC DSS was also identified, either to improve accessibility for multiple users or to integrate supply chain partners. [START_REF] Stevenson | Refining a workload control (WLC) concept: A case study[END_REF]Hendry (2007a and2007b) explored the implications of web-functionality for WLC while [START_REF] Silva | Production planning in a make to order environment: A case of a mould manufacturer[END_REF] and [START_REF] Silva | MAPP -A web-based decision support system for the mould industry[END_REF] developed a system that incorporated this technology. Webfunctionality can be considered a step towards integration into the wider supply chain and integration with other systems, e.g., Enterprise Resource Planning (ERP) systems but previous studies had not explicitly considered this. A further conceptual extension is provided by [START_REF] Soepenberg | The order progress diagram: A supportive tool for diagnosing delivery reliability performance in make-to-order companies[END_REF] who introduced a diagram which allows order progress to be tracked in a simple graphical way, helping to diagnose the causes of, and control, lateness. The tool was applied by [START_REF] Land | Production planning and control in SMEs: Time for change[END_REF] to identify the causes of PPC implementation problems in seven cases. The main contribution to Group 4 was by [START_REF] Breithaupt | The workload control concept: Theory and practical extensions of load oriented order release[END_REF] who made several refinements to LOOR and LOMC; for example, a dialogue-oriented extension to overcome balancing problems described by [START_REF] Wiendahl | Anwendung der Belastungsorientierten Fertigungssteuerung[END_REF] and a logistic operating curve to define optimal parameters [START_REF] Nyhuis | Logistische Kennlinien: Grundlagen, Werkzeuge und Anwendungen[END_REF].

Finally, Table 1 summarises the most important conceptual WLC studies from the last three decades according to the categorisation introduced at the beginning of subsection 4.1.1.

[Take in Table 1]

Conceptual Research: Future Research Directions

After 30 years, WLC is now a mature concept suitable as either a comprehensive PPC approach (e.g., Land & Gaalman, 1996a;[START_REF] Stevenson | Refining a workload control (WLC) concept: A case study[END_REF] or an interface between a higher level planning system and the shop floor (e.g., [START_REF] Bechte | Load-oriented manufacturing control just-in-time production for job shops[END_REF][START_REF] Breithaupt | The workload control concept: Theory and practical extensions of load oriented order release[END_REF]. But to remain at the • Developing a comprehensive framework to clearly outline the characteristics of WLC and delimit it from other PPC systems, such as ConWIP.

• Exploring how WLC can be incorporated into (more) ERP systems. While [START_REF] Fandel | PPS-und integrierte betriebliche Softwaresysteme Grundlagen, Methoden, Marktanalyse[END_REF] reported that LOOR is included in 28% of commercially available PPC and ERP systems, up-todate statistics are not available. Nor is it clear whether recent advances in the WLC literature have been incorporated. However, convincing more ERP vendors to adopt WLC may rely on establishing further empirical evidence of its positive effect on performance.

• Developing WLC to integrate the concept further into the management of supply chains (e.g., through more sophisticated web functionality). Research (1980Research ( -1990) ) Few analytical research contributions were made in the 1980s and 1990s because an adequate approach for modelling WLC was missing; all of the contributions that did emerge were based on queuing theory. The first attempt was by [START_REF] Kanet | Load-limited order release in job shop scheduling systems[END_REF] who used a single-machine model to analyse the influence of load limited order release on shop performance. The author found that it may negatively influence performance but this could be due to the simplicity of the release method applied. A second contribution was made as part of the conceptual study by Hendry & Kingsman (1991b), who analysed the relationship between the Released Backlog Length (RBL) and throughput time and the influence of the percentage of priority orders on the performance of nonpriority orders. The work is similar to a simulation study by [START_REF] Malhotra | Management of vital customer priorities in job shop manufacturing environments[END_REF] -the same results were obtained but much quicker and without building a complex simulation model; this demonstrated the potential of analytical modelling. Finally, [START_REF] Missbauer | Order release and sequence-dependent set-up times[END_REF] studied the influence of sequence-dependent set-up times on the relationship between WIP and throughput showing that when sequence-dependent set-up times exist, throughput may be improved by increasing WIP because the number of set-ups decreases if more jobs are waiting in front of a work centre and can be grouped together. Research (2000Research ( -2009) ) Few analytical research contributions were made in the 1980s or 1990s but there have been several recent attempts. Contributions are divided into three groups: (1) analytical models applying queuing (2) mathematical analysis of new release methods; and, (3) analytical tools to facilitate management decisions. In Group 1, [START_REF] Haskose | Modelling flow and jobbing shops as a queuing network for workload control[END_REF] developed a tandem queuing network with buffer constraints corresponding to a pure flow shop. This was extended by [START_REF] Haskose | Performance analysis of make-to-order manufacturing systems under different workload control regimes[END_REF] to an arbitrary queuing network with buffer constraints corresponding to a general flow shop and a pure job shop; however, only an approximate solution for the arbitrary queuing network could be provided. While this work is important to analytical model building in WLC research, it remains unclear whether applying buffer constraints is appropriate as most WLC policies do not restrict the buffer (or queue length) in front of work centres; work centre buffers are usually considered infinite as the buffering happens in the pre-shop pool to avoid blocking on the shop floor. An alternative was provided by Missbauer (2002a and[START_REF] Missbauer | Models of the transient behaviour of production units to optimize the aggregate material flow[END_REF] who used the theory of transient queuing networks to build aggregate order release planning models, introducing a clearing function model with more than one independent variable. This appears more appropriate, but clearing function models are based on steady-state assumptions and hence still only provide approximation solutions. An additional contribution was made by Missbauer (2002b), where a single-stage model based on open queuing networks was introduced to explore the influence of lot sizes on WLC. [START_REF] Enns | Evaluating shop floor input control using rapid modelling[END_REF] made the main contribution to Group 2 by proposing Minimum Release Time Interval (MRTI), a method which releases jobs from the input buffer at equal time intervals corresponding to the expected processing time of a job at the bottleneck. MRTI is analysed using rapid modelling which provides an insight into performance without building a simulation model; the drawback is that feedback cannot be modelled. Therefore, an additional simulation model was built to validate the results and compare MRTI with alternatives. Further tests showed that MRTI did not perform as well as some sophisticated traditional order release methods. Hence, it remains unclear whether effective new release methods can be developed using analytical modelling in isolation. The main contributions to Group 3 are [START_REF] Kingsman | Modelling input-output workload control for dynamic capacity planning in production planning systems[END_REF], who proposed a mathematical model to facilitate dynamic capacity planning at the customer enquiry stage, and [START_REF] Corti | A capacity-driven approach to establish reliable due dates in a MTO environment[END_REF] who presented a heuristic to verify the feasibility of DDs requested by customers. However, while [START_REF] Corti | A capacity-driven approach to establish reliable due dates in a MTO environment[END_REF] provided a first step towards providing managers with an effective tool for making fast and appropriate decisions, the focus was purely on checking the feasibility of proposed DDs and capacity planning at the customer enquiry stage; other important issues, such as the process of actually proposing a DD and parameter setting at the order release stage (e.g. workload norms), were neglected.

Analytical Research

Analytical

Analytical

Finally, Table 2 summarises the most important analytical WLC research contributions from the last three decades demonstrating the increased interest in this approach in the last decade.

Analytical Research: Future Research Directions

Analytical research has grown substantially and positive progress has been made in modelling WLC; future research directions include:

• Going beyond the approximate analytical modelling solutions presented to date.

• Developing simpler, yet effective, heuristics and models to support managers in making faster decisions in practice, including tools to support the process of setting appropriate WLC parameters. 4.3.1 Empirical Research (1980-1990) Three types of empirical research were conducted in the 1980s and 1990s: (1) research based on single cases; (2) research based on multiple cases; and, (3) single case study accounts of hybrid PPC systems. Successful implementations of LOMC and LOOR were reported in Group 1 by [START_REF] Bechte | Theory and practise of load-oriented manufacturing control[END_REF] and [START_REF] Bechte | Load-oriented manufacturing control just-in-time production for job shops[END_REF] and in Group 2 by [START_REF] Wiendahl | Application of load-oriented manufacturing control in industry[END_REF]. All three report on implementations in small and medium sized MTO companies (from plastic and textile processing [START_REF] Bechte | Theory and practise of load-oriented manufacturing control[END_REF] to mechanical engineering [START_REF] Wiendahl | Application of load-oriented manufacturing control in industry[END_REF][START_REF] Bechte | Load-oriented manufacturing control just-in-time production for job shops[END_REF]), reporting reductions in lead times and WIP. Further empirical studies categorised in Group 1, where implementation success was less conclusive, were presented by Bertrand & Wortman (1981), [START_REF] Tatsiopoulos | A microcomputer-based interactive system for managing production and marketing in small component manufacturing firms using a hierarchical backlog control and lead time management technology[END_REF], [START_REF] Fry | A procedure for implementing input/output control: A case study[END_REF], [START_REF] Hendry | A decision support system to manage delivery and manufacturing lead times in make to order companies[END_REF], and Hendry et al. (1993). Finally, research in Group 3 emerged at the end of the 1990s when [START_REF] Park | Delivery date decision support system for the large scale make to order manufacturing companies: A Korean electric motor company case[END_REF] implemented customer enquiry management theory from the LUMS Approach but without the order release rule. A hybrid system was built that retained the company's existing releasing policy. The authors developed a Decision Support System (DSS) incorporating a Heuristic Delivery Date Decision Algorithm (HDDDA) that revised the capacity planning model within the LUMS Approach. The system helped managers set feasible DDs but only considered the current load of the bottleneck machine and hence may be susceptible over time to the 'wandering bottleneck' problem (see [START_REF] Lawrence | Shifting production bottlenecks: causes, cures, and conundrums[END_REF]. The work demonstrated the flexibility of the LUMS Approach (elements of the theory could be combined with existing business processes) and the hybrid system improved the performance of the company.

Empirical Research

By the end of the 1990s, the body of empirical research was limited and papers tended to focus on reporting the before and after situation in the cases without describing the process of implementation itself. The exception to this was [START_REF] Fry | A procedure for implementing input/output control: A case study[END_REF] who provided a framework for Research (2000Research ( -2009) ) While empirical research in the 1980s and 1990s focused on comparing performance before and after implementation with the researcher as an external observer, recent contributions have focused more on the process of implementation with the researcher participating in organisational change.

Empirical

Hence, the scope of empirical WLC research has extended to action research; like in the 1980s and 1990s, research is divided into three groups: (1) research based on single cases; (2) research based on multiple cases; and, (3) single case study accounts of hybrid PPC systems.

Group 1 consists of [START_REF] Stevenson | Refining a workload control (WLC) concept: A case study[END_REF] and [START_REF] Silva | MAPP -A web-based decision support system for the mould industry[END_REF]; both include a WLC DSS based on the LUMS Approach. The former was implemented in a small MTO company in the UK and the latter in a medium sized mould-producing MTO company in Portugal. [START_REF] Stevenson | Theoretical development of a workload control methodology: Evidence from two case studies[END_REF] then collaborated to compare the two cases while research questions raised by the implementation in the UK (and an additional case in the Netherlands) were summarised in [START_REF] Hendry | Investigating implementation issues for workload control (WLC): A comparative case study analysis[END_REF].

One of these concerned how assembly and rush orders could be accommodated; this has since been partially addressed by [START_REF] Thürer | Workload control release mechanisms: From practice back to theory building[END_REF] who used simulation to find that prioritizing rush orders at the release stage is the best solution. This group of research has outlined implementation problems (not just results) and outstanding research questions. In time, additional responses to that provided by [START_REF] Thürer | Workload control release mechanisms: From practice back to theory building[END_REF] are expected. Finally, none of the authors in Group 1 and 2 who presented positive empirical results in the 1980s and 1990s have presented follow-up results since 2000 which demonstrate whether or not success was sustained over a long period of time.

In Group 2, [START_REF] Land | Production planning and control in SMEs: Time for change[END_REF] explored why PPC concepts regularly fail by analysing data from seven companies so future research can use the insight to implement WLC principles in practice. Key problems were uncontrolled delays in engineering and inadequate capacity planning overviews to support sales decisions. The former could be accounted for within the order entry/preproduction stage of WLC while the latter can be overcome by applying WLC principles as shown in the work of [START_REF] Park | Delivery date decision support system for the large scale make to order manufacturing companies: A Korean electric motor company case[END_REF] and [START_REF] Riezebos | Improving a practical DBR buffering approach using workload control[END_REF] below.

In Group 3, [START_REF] Riezebos | Improving a practical DBR buffering approach using workload control[END_REF] demonstrated that WLC can be successfully implemented when part of a hybrid system. Like [START_REF] Park | Delivery date decision support system for the large scale make to order manufacturing companies: A Korean electric motor company case[END_REF], [START_REF] Riezebos | Improving a practical DBR buffering approach using workload control[END_REF] maintained the order release rule already used in the company (Drum-Buffer-Rope) and restructured order acceptance from a procedure where the sales department was allowed to accept orders freely up to a maximum financial daily turnover limit to a capacity-based approach considering two semi-interchangeable bottleneck machines. The authors also introduced LOMC principles, rather than the LUMS Finally, Table 3 summarises the most important empirical WLC research contributions of the last three decades.

[Take in Table 3]

Empirical Research: Future Research Directions

Recent empirical research has provided an insight into the implementation problems encountered in practice and raised questions regarding how they can be overcome, potentially leading to new conceptual advances. The future of WLC appears to lie in a comprehensive PPC system based on the LUMS and LOMC approaches but in which independent order release rules may be embedded.

Future research directions include:

• Continuing to focus on implementation challenges and the process of implementation itself so future research can identify solutions to problems identified. This may also lead to developing a clear implementation strategy or roadmap for WLC.

• Considering the sustainability of implementation success over time. WLC implementations should be revisited several years after implementation to observe if the concept is still being used (or how it has been adapted over time) and determine how any positive effects can be sustained. 4.4.1 Simulation-Based Research (1980-1990) Simulation was the dominant approach in the WLC literature in the 1980s and 1990s. Four groups of simulation-based research can be identified: (1) testing the influence of WLC (mostly ORR) on performance to find the best fit between control stages; (2) developing new release methods and comparing performance; (3) studying the influence of environmental (external) parameters on performance; and, (4) analysing the influence of WLC characteristics (internal parameters) on performance.

Simulation-Based Research

Research in Group 1 was concerned with evaluating different combinations of DD, order release and dispatching rules to determine the best combination. Bertrand (1983a) and [START_REF] Baker | The effects of input control in a simple scheduling model[END_REF] tested the influence of controlled order release on performance while [START_REF] Ragatz | An evaluation of order release mechanisms in a job shop environment[END_REF] sought to find the best fit between dispatching and job release rules. This research continued throughout the 1990s (e.g., [START_REF] Ahmed | Due date assignment, job order release and sequencing interaction in job shop scheduling[END_REF][START_REF] Wein | A broader view of the job-shop scheduling problem[END_REF][START_REF] Fredendall | Information and scheduling in a dual resource constrained job shop[END_REF] but a combination of rules which clearly performs best under all conditions could not be determined. In an attempt to make the different control stages work together, authors such as [START_REF] Melnyk | Load smoothing by the planning and order review/release systems: A simulation experiment[END_REF], [START_REF] Park | Load smoothing with feedback in a bottleneck job shop[END_REF] and [START_REF] Salegna | Integrating the planning and scheduling systems in a job shop[END_REF] introduced "load smoothing" to control the entry of jobs into the pool. A ceiling (upper bound) and floor (lower bound) limit for the pool was introduced and the load was either pulled forward or pushed backward to smooth the overall pool load and improve order release performance. Melnyk et al. (1994b) later found that this adversely affected dispatching performance; hence, no conclusive results emerged and this research stagnated towards the end of the 1990s.

Researchers in Group 2 compared and developed new order release rules, such as: load balancing and load limiting [START_REF] Shimoyashiro | Input scheduling and load balance control for a job shop[END_REF]; Starvation Avoidance (SA: [START_REF] Glassey | Closed-loop job release control for VLSI circuit manufacturing[END_REF]; Superfluous Load Avoidance Release (SLAR: [START_REF] Land | The performance of workload control concepts in job shops: Improving the release method[END_REF]; and, the Path Based Bottleneck (PPB) approach [START_REF] Philipoom | An evaluation of capacity sensitive order review and release procedures in job shops[END_REF]. In addition, the conceptual work by [START_REF] Tatsiopoulos | Lead time management[END_REF] led to a control system presented by [START_REF] Onur | An input/output control system for the dynamic job shop[END_REF] while [START_REF] Hendry | Alternative order release mechanisms: a comparison by simulation[END_REF] tested the order release policy introduced by Hendry & Kingsman (1991a). Simulation was also used to compare WLC release policies against each other (e.g., [START_REF] Sabuncuoglu | Analysis of order review/release problems in production systems[END_REF] or against the release policies of other PPC systems, such as ConWIP [START_REF] Roderick | A comparison of order release strategies in production control systems[END_REF][START_REF] Lingayat | An order release mechanism for a flexible flow system[END_REF]. However, none of these studies were able to establish one universal rule which performed best under all performance measures. By the end of the 1990s, an extensive set of alternative order release mechanisms had been developed and research in this group began to stagnate.

Researchers in Group 3 studied the influence of environmental (external) parameters, e.g., worker flexibility or sequence-dependent set-up times, on the performance of combinations of DD, order release and dispatching rules. For example, Park & Bobrowski (1989) and Bobrowski & Park (1989) showed that flexible workers have a positive effect on shop floor performance, [START_REF] Philipoom | Capacity-based order review/release strategies to improve manufacturing performance[END_REF] demonstrated that rejecting a small proportion of orders can improve performance, while [START_REF] Malhotra | Management of vital customer priorities in job shop manufacturing environments[END_REF] found that the number of orders given priority should not exceed 30% or the performance of non-priority orders will deteriorate significantly. Finally, [START_REF] Philipoom | Order review/release in the absence of adherence to formal scheduling policies[END_REF] showed that order release can offset performance losses that occur when operators refuse to follow dispatching rules. Each of these studies focused on an individual environmental parameter but, in practice, researchers encounter complex combinations of factors.

Research in Group 4 emerged towards the end of the 1990s. [START_REF] Cigolini | Comparison of Order Review and Release techniques in a dynamic and uncertain job shop environment[END_REF] underlined the importance of testing the characteristics of release rules (internal parameters) iteratively, i.e., gradually changing them to determine applicability to different contexts. The authors analysed the influence of workload accounting over time approaches on performance and emphasised the importance of robustness in dynamic and uncertain job shop environments; probabilistic approaches performed the best. Perona & Portioli (1998) releases (check period) and the planning period on the performance of LOOR. The authors suggested that the check period should be smaller than the planning period but exact values depend on the average processing time. The authors did not present a definitive answer as to how all of the internal parameters relevant to WLC should be set -an important issue for research in the 2000s. Simulation-Based Research (2000[START_REF] Missbauer | Models of the transient behaviour of production units to optimize the aggregate material flow[END_REF] Simulation remains the dominant method adopted in WLC research. The same four groups of research noted in the 1980s and 1990s are evident since 2000 but with changing importance and objectives. The only studies which continue research in Group 1 are [START_REF] Weng | Multi-agent-based workload control for make-to-order manufacturing[END_REF] and [START_REF] Moreira | A methodology for planning and controlling workload in a job-shop: a four-way decision-making problem[END_REF]. [START_REF] Weng | Multi-agent-based workload control for make-to-order manufacturing[END_REF] presented a multi-agent WLC methodology consisting of a network of four independent agents, one for each of the three ORR control stages and one for information feedback. Previous research had struggled to cope with interaction between the different control levels but the network allows all levels to be controlled simultaneously. Results suggested that dynamic control might be a better solution than trying to find a best-fit combination of rules. Like many authors in the 1980s and 1990s, [START_REF] Moreira | A methodology for planning and controlling workload in a job-shop: a four-way decision-making problem[END_REF] struggled to find one best-fit combination for the different control stages.

4.4.2

The previous two decades had provided an almost exhaustive set of release methods; as a result, few attempts to add to this list have been made since 2000 and the number of contributions to research in Group 2 has significantly decreased. [START_REF] Sabuncuoglu | A load-based and due-date-oriented approach to order review/release in job shops[END_REF] developed the DD and Load-oriented Release (DLR) method to minimise the Mean Absolute Deviation (MAD) of lateness by considering both DDs and shop load. DLR outperformed several alternatives, e.g., the Periodic Aggregate Loading (PAGG) and Path Based Bottleneck (PBB) methods including in terms of MAD and throughput time. [START_REF] Enns | The effectiveness of input control based on aggregate versus bottleneck workloads[END_REF] developed the Aggregate Load Oriented Release (ALOR) and Bottleneck Load Oriented Release (BLOR) methods. ALOR performs best in a flow shop but is outperformed if the flow characteristics are less structured. But none of these new rules have been applied by other authors, arguably because they are only slight variants on previously existing, and adequately performing, rules. Finally, [START_REF] Fredendall | Concerning the theory of workload control[END_REF] compared WLC order release rules, and rules from other PPC systems, concluding that no single rule performs best under all conditions; the findings supported those made by authors in the 1980s and 1990s. Within Group 3, [START_REF] Oosterman | The influence of shop characteristics on workload control[END_REF] and [START_REF] Land | Workload control in job shops: Grasping the tap[END_REF] [START_REF] Silva | MAPP -A web-based decision support system for the mould industry[END_REF] and [START_REF] Stevenson | Theoretical development of a workload control methodology: Evidence from two case studies[END_REF]. Giving priority to large jobs at the release stage significantly improved the performance of large jobs with only a small performance loss for small jobs. A further implementation issue experienced by [START_REF] Silva | MAPP -A web-based decision support system for the mould industry[END_REF] was how to group machines into work centres. This had been partly addressed earlier by Henrich et al. (2004b); the authors sought to reduce feedback requirements from the shop floor (a significant problem in practice) and found that this could be achieved by grouping machines with similar processing capabilities into work centres and controlling the load of the work centre rather than each individual machine. While information feedback was reduced, results indicated that the smaller the work centre (approaching one machine per centre) the better the performance. Hence, a trade-off has to be made between the cost of investing in efficient data collection tools and the performance loss of intermittent feedback.

Grouping interchangeable machines allows the allocation of jobs to a particular machine to be delayed until the last possible moment; however, machines are often semi-interchangeable, restricting flexibility. [START_REF] Henrich | Grouping machines for effective workload control[END_REF]2007) found that the routing decision between two semi-interchangeable machines has to be made as late as possible if optimum performance is to be achieved. This is consistent with [START_REF] Kim | Evaluating order release mechanisms in a job shop with sequence-dependent setup times[END_REF] who studied the influence of sequence dependent set-up times. If jobs have to wait for a free machine, or set-up times depend on shortterm sequencing decisions, then the dispatching rule determines shop floor performance. This is contrary to the many authors who had earlier suggested that if order release is controlled, only a simple dispatching rule is necessary.

Further research into handling sequence-dependent set-up times and routing decisions for semiinterchangeable machines at the order release stage is required, as is research into handling assembly orders. When considering the parts which make up an assembly order, should all parts be released together or treated independently? Precedence rules within the product structure also influence how the job flows through the shop floor, further complicating how workload might be accounted for over time. [START_REF] Bertrand | An investigation of order release and flow time allowance policies for assembly shops[END_REF] provided a starting point for integrating assembly orders into WLC by testing several order release policies. Results suggested that performance is not affected by releasing all the work orders of an assembly order at the same time compared to treating them independently. Moreover, average lateness for assembly orders can be reduced to zero by planning all work orders of an assembly order with a flow time allowance (used to forward or backward schedule the orders) equal to the average operation waiting time. However, the authors did not apply any workload limit thereby avoiding the workload accounting problem and meaning that their contribution cannot strictly be considered part of the WLC literature.

Another important factor missing in WLC simulation research is the 'human factor'; the only [START_REF] Bertrand | Workload based order release and productivity: A missing link[END_REF]. The authors concluded that the level of WIP influences worker productivity and thus processing times. The authors argued that an optimum WIP level can be found and that WLC can be an appropriate means of maintaining WIP at the optimal level. Incorporating human factors like this within WLC research is important but can only be achieved by combining simulation models with empirical experience.

Finally, in Group 4, [START_REF] Cigolini | An experimental investigation on workload limiting methods with ORR policies in a job shop environment[END_REF] continued the work of Perona & Portioli (1998) and [START_REF] Cigolini | Comparison of Order Review and Release techniques in a dynamic and uncertain job shop environment[END_REF] by investigating the influence of different workload bounding policies on performance. The authors found that an upper and a lower bound might conflict each other and negatively affect release performance, leading to one of the conceptual refinements made by [START_REF] Stevenson | Aggregate load oriented workload control: A review and a reclassification of a key approach[END_REF]. [START_REF] Kingsman | The relative contributions of input and output controls on the performance of a workload control system in Make-To-Order companies[END_REF] studied the influence of input and output control on the performance of the LUMS Approach. A first simulation applied only input control while a second applied input and output control; results suggested that the two control mechanisms complement each other. Finally, [START_REF] Land | Workload control in job shops: Grasping the tap[END_REF] explored the influence of the check period, shop floor characteristics and flow time allowance on the performance of order release rules, summarising the results in [START_REF] Land | Parameters and sensitivity in workload control[END_REF]. No further contributions have been made since [START_REF] Land | Workload control in job shops: Grasping the tap[END_REF]2006), arguably because most key parameters have now been studied.

Findings should assist practitioners in setting WLC parameters but empirical evidence which verifies this is required.

Finally, Table 4 summarises the most important simulation-based WLC studies from the last three decades. The table highlights the clear shift away from research in Group 1 and 2 and the increase in research in Group 3 and 4, as discussed earlier in this section.

[Take in Table 4] Table 5 summarises simulation properties from papers since 2000, including the way jobs are ordered in the pool, the order release rule, performance criteria and approach to statistically validating results. Almost all use a special time-related policy to consider jobs for release, generally either backward or forward scheduled release or by considering the job with the earliest (planned) release date or earliest DD first. Many release rules have been simulated; however, in the last decade, the approaches outlined in Section 3.2 have prevailed (probabilistic and aggregate load approaches). The performance measures are either time-related (e.g. throughput times or lateness) or according to the number of jobs. Cost measures are less common in recent studies, perhaps because of the subjective nature of cost estimates in simulations; future research should consider how cost measures can be incorporated in an objective manner. Finally, the statistical analysis of [Take in Table 5] Table 6 summarises the shop floor characteristics from papers since 2000, including routing sequence and length, processing times, arrival time of jobs, number of work centres, whether the shop floor is hypothetical or a real-life shop, and the simulation software used. Most studies are based on similar shop floor configurations to those presented by [START_REF] Melnyk | Order review/release: research issues and perspectives[END_REF], simulating a pure job shop with uniformly distributed routing lengths, a fixed mean processing time which follows a certain distribution, and an arrival time adapted to achieve a certain utilisation level. Few studies base shop floor configuration on a real-life shop floor; although these would arguably provide the more realistic insight, a hypothetical configuration allows individual parameters to be studied while other parameters are controlled. Several simulation software packages have been used; authors do not routinely provide information about the logic underpinning the models developed, making it hard to compare results across researchers reliably.

[Take in Table 6] • Determining how to best handle assembly orders; while [START_REF] Bertrand | An investigation of order release and flow time allowance policies for assembly shops[END_REF] provided a starting point, more research is required.

• Developing more realistic simulation models; most are hypothetical and, in many ways, do not reflect reality [START_REF] Perona | Workload control: A comparison of theoretical and practical issues through a survey in field[END_REF] leading to problems when researchers attempt to implement the results in practice. This should include incorporating more human factors within the design of simulation experiments.

• Validating refinements to the WLC concept (see Section 4.1.2). This would combine empirical and simulation-based research to improve the conceptual basis of WLC.

• Providing an open-source WLC model. If all researchers used the same simulation model, results could be compared across research groups more reliably and the time spent on model building would be reduced. This could apply to code for order release or dispatching rules and for shop and job characteristics.

Conclusion

This review began by considering how the field of WLC has evolved towards identifying how it should evolve in the future. A comprehensive systematic review of the conceptual, analytical, empirical and simulation-based WLC literature published since 1980 has been conducted. In response to Research Question 1, regarding the evolution of the field of WLC, the following conclusions could be drawn:

• By the end of the 1990s, the conceptual development of the LUMS Approach and LOMC had reached maturity; the focus since 2000 has shifted towards conceptual refinement, e.g., in light of empirical evidence.

• There has been a substantial increase in analytical modelling since 2000 while the focus of field research has shifted from observation, and reporting before/after implementation, to focusing on how WLC can be implemented through participation.

• While it remains the most commonly adopted method, simulation has somewhat declined in use and its focus has shifted from finding the best fit between DD setting, release and dispatching rules to internal parameter setting and the influence of external parameters on the performance of order release rules, in many cases addressing issues encountered during empirical research.

Many valuable contributions to the development of WLC have been presented in the past three decades; however, there are many opportunities for further research. To conclude this paper, and in response to Research Question 2, outstanding WLC research gaps identified include:

• Conceptual Research: the need to give far greater consideration to human factors in the design of PPC systems based on WLC; and, the need to integrate WLC with ERP systems and the wider supply chain.

• Analytical Research: the need to develop tools that support managers in making fast and appropriate decisions, e.g., during the process of setting appropriate (internal) WLC parameters.

• Empirical Research: the need to conduct further action research into how WLC can be effectively implemented in practice; and, to investigate whether improvements can be sustained over time.

• Simulation Based Research: the need to further improve simulation models, including studying human factors that affect WLC; and, feeding back empirical findings to simulation-based WLC research to improve the applicability of WLC theory to real-life job shops.

List of Tables & Figures

Tables Table 1: Summary of Conceptual WLC Research (1980[START_REF] Missbauer | Models of the transient behaviour of production units to optimize the aggregate material flow[END_REF] (Land & Gaalman, 1996a)

  more than once were chosen for the final WLC database. The final database contained 107 articles (27 from the 1980s, 42 from the 1990s and 38 since 2000). All articles in the final database have been included in the systematic review which is presented in what follows.

  ; but this has since been shown to perform poorly in simulation

  concept has to evolve with contextual changes and new technologies. Future research directions include:

  a simple I/OC system and[START_REF] Wiendahl | Load oriented manufacturing control[END_REF] who included a 6-stage implementation framework.

  [START_REF] Park | Delivery date decision support system for the large scale make to order manufacturing companies: A Korean electric motor company case[END_REF], with a positive impact on performance.

  investigated the influence of the time between two

  studied the influence of routing direction on the performance of WLC. The studies investigated four particular shop configurations (pure and restricted job shops and pure and general flow shops) showing the superior performance of the corrected aggregate load approach if a dominant routing direction exists.[START_REF] Thürer | Workload control release mechanisms: From practice back to theory building[END_REF] of job size on performance, addressing a research question raised by

  and should be developed in the future.

  4.4.3 Simulation-BasedResearch: Future Research Directions Recent research has shifted the focus from testing release mechanisms to addressing practical questions emerging from implementation experience; only 5 of the 15 simulation studies published since 2000 focused on release method development and comparison. Future research directions should include the following:

FiguresFigure 1 :

 1 Figures

  Figure 1: (a) ORR and ORR WLC (Land & Gaalman, 1996a); (b) LUMS Approach to WLC

  assessed the relevance of PPC concepts to MTO companies, concluding that LOMC and what later became known as the LUMS Approach were most appropriate. Researchers in the third group focused on developing the LUMS Approach.

	F o r
	P
	e
	e r
	R
	e
	v i e
	w
	O n l
	y
	7

Table 2 :

 2 Summary of Analytical WLC Research (1980[START_REF] Missbauer | Models of the transient behaviour of production units to optimize the aggregate material flow[END_REF]

Table 3 :

 3 Summary of Empirical WLC Research (1980[START_REF] Missbauer | Models of the transient behaviour of production units to optimize the aggregate material flow[END_REF]

Table 4 :

 4 Summary Simulation-Based WLC Research (1980[START_REF] Missbauer | Models of the transient behaviour of production units to optimize the aggregate material flow[END_REF]

Table 5 :

 5 Summary of Simulation Properties

Table 6 :

 6 Summary of Simulated Shop Floor Characteristics

Table 1 :

 1 ; (b) LUMS Approach to WLC Summary ofConceptual WLC Research (1980[START_REF] Missbauer | Models of the transient behaviour of production units to optimize the aggregate material flow[END_REF]

	Group	1980s	1990s	2000s
	Group 1: Categorisation of WLC	None	Bergamaschi et al. (1997) Wisner (1995)	Henrich et al. (2004a)
	Group 2: Reviewing different PPC concepts and WLC	Hendry & Kingsman (1989)	Land & Gaalman (1996a) Zäpfel & Missbauer (1993b)	Stevenson et al. (2005) Fowler et al. (2002)
				Stevenson et al. (2009)
			Kingsman et al. (1996)	Hendry et al. (2008)
			Hendry & Kingsman	Soepenberg et al. (2008)
	Group 3: Developing the	Kingsman et al. (1989)	(1993)	Stevenson & Silva (2008)
	theory of the LUMS Approach F o Tatsiopoulos & Kingsman (1983) r Group 4: Developing the theory of LOMC Bechte (1988) 1	Kingsman et al. (1993) Hendry & Kingsman (1991a,b) Bechte (1994) 1	Stevenson & Hendry (2007a,b) Stevenson (2006) Stevenson & Hendry (2006) Breithaupt et al. (2002)
		P
		e
		e r
		R
		e
		v i e
			w
			O n l
				y
		2

1 ) Conceptual and empirical research

Table 2 :

 2 Summary of Analytical WLC Research (1980[START_REF] Missbauer | Models of the transient behaviour of production units to optimize the aggregate material flow[END_REF]

	Group	1980s	1990s	2000s
	Group 1: Analytical models applying queuing theory	Kanet (1988)	Missbauer (1997)	Missbauer (2009) Haskose et al. (2004) Haskose et al. (2002) Missbauer. (2002a 1 ,b)
	Group 2: Mathematical
	analysis of new release	None	None	Enns (2000)
	methods
	Group 3: Analytical tools to facilitate management decisions	None	None	Corti et al. (2006) Kingsman (2000)

1 ) Analytical and simulation based research

Table 3 :

 3 Summary of Empirical WLC Research (1980[START_REF] Missbauer | Models of the transient behaviour of production units to optimize the aggregate material flow[END_REF]

	Group	1980s	1990s	2000s
	Group 1: Research based	Bechte (1988) 1	Bechte (1994) 1	Silva (2006)
	on single cases	Fry & Smith (1987)	Hendry et al. (1993)	Stevenson (2006)
	Group 2: Research based on multiple cases	None	Wiendahl (1992)	Land & Gaalman (2009)
	Group 3: Single case
	study accounts of hybrid	None	Park et al. (1999)	Riezebos et al. (2003)
	PPC systems
	1 ) Conceptual and empirical research

Table 4 :

 4 Summary ofSimulation-Based WLC Research (1980[START_REF] Missbauer | Models of the transient behaviour of production units to optimize the aggregate material flow[END_REF]

	Group	1980s	1990s	2000s
			Fredendall et al. (1996)
	Group 1: Testing the influence of WLC on performance to find the best fit between control	Ragatz & Mabert (1988) Baker (1984) Bertrand (1983a)	Salegna (1996) Park & Salegna (1995) Melnyk et al. (1994) Ahmed & Fisher (1992)	Moreira & Alves (2009) Weng et al. (2008)
	stages		Wein & Chevalier (1992)
			Melnyk et al. (1991)
			Sabucuoglu & Karapinar
	Group 2: Developing new release methods and comparing performance	Glassey & Resende (1988) Onur & Fabrycky (1987) Shimoyashiro (1984)	(1999) Land & Gaalman (1998) Lingayat et al. (1995) Hendry & Wong (1994) Philipoom et al. (1993)	Fredendall et al. (2009) Enns & Prongue Costa (2002) Sabuncuoglu & Karapinar (2000)
			Roderick et al. (1992)
				Thürer et al. (2009)
				Henrich et al. (2007)
	Group 3: The influence of environmental (external) parameters on performance	Bobrowski & Park (1989) Park & Bobrowski (1989)	Philipoom & Fry (1999) Malhotra et al. (1994) Philipoom & Fry (1992)	Henrich et al. (2006) Henrich et al. (2004b) Bertrand & Van Ooijen (2002)
				Missbauer (2002a) 1
				Oosterman et al. (2000)
	Group 4: The influence of WLC characteristics (internal parameters) on performance	None	Cigolini et al. (1998) Perona & Portioli (1998)	Land (2006) Cigolini & Portioli-Staudacher (2002) Hendry & Kingsman (2002)
	1 ) Analytical and simulation based research

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 5 Summary of Simulation Properties

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) [a-b] = between a and b;

2 ) [a] = mean a;

3 ) arrival time such that utilisation at this level; (Land & Gaalman, 1996a)