Lanouar Charfeddine
email: lanouar_charf@yahoo.fr.

Dominique Guegan

IS IT POSSIBLE TO DISCRIMINATE BETWEEN DIFFERENT SWITCHING REGRESSIONS MODELS: AN EMPIRICAL INVESTIGATION?

Keywords: JEL classication: C12, C15, F31 Switching Models, Sup LR test, Empirical power, Exchange rate

 and the Threshold Auto-Regressive Models (TAR) of Lim and Tong (1980). This work is motivated by the fact that generally practicians use, in applications, switching models without any statistical justication. Using experiment simulations, we show that it is very dicult to discriminate between the MSAR and the SE-TAR models specially using large samples. This means that when the null hypothesis is rejected, it appears that dierent switching models are signicant. Moreover, the results show that the power of the sup LR test is sensitive to the mean, the noise variance and the delay parameter.

Finally, we apply this methodology to two time series: the US GNP growth rate and the US/UK exchange rate. We shall retain a Markov switching process for the US GNP growth rate and the US/UK exchange rate (monthly data). For the US/UK exchange rate (quarterly data), we accept the null hypothesis of a random walk.

INTRODUCTION:

In the last two decades, a huge literature on non linear models have been developed. Researchers have largely used this class of models to explain many macroeconomic and nancial specic phenomena like sudden changes, irreversibility and asymmetry... From the available non linear models, a specic attention have been attributed to the Markov Switching Auto-Regressive models (MSAR afterhere) of [START_REF] Hamilton | A new Approach to the Economic Analysis of Non stationarity Times Series and the Business Cycle[END_REF] and the SETAR model of Lim and Tong (1980).

Since these two previous seminal papers, applications of the MSAR and the SETAR models have been extensively increased. Their use in empirical applications is motivated by many economics and internationals event like the collapse of the Bretton Wood's system, the two oil crisis, the Plaza Accord, the collapse of the European Monetary System (EMS) and the intervention of Banks in foreign exchanges market. In empirical applications, the Markov switching models have been employed to study the aggregate outputs [START_REF] Hamilton | Analysis of Time Series Subject to Change in Regime[END_REF], the annual growth rate of consumption in an asset-pricing (Cecchetti et al., 1990), the behavior of foreign exchanges rates (Engle and Hamilton, 1989), the eect of federal reserve actions on interest rates (Hamilton 1988, Garcia andPerron 1989), for instance. In the other hand, many others researchers have used the SETAR models in empirical applications, we refer to [START_REF] Potter | A Nonlinear Approach to US GNP[END_REF], [START_REF] Hansen | Inference in TAR models[END_REF], Proietti (1998) and Ferrara and Guégan (2005) for instance.

These two models have the advantage of being able to model and to capture asymmetry, sudden changes and irreversibility time observed in many economic and nancial time series. Despite these similarities, these models have been involved, in the literature, largely independently. Moreover, testing between these two types of switching regression models are seldom explored by researchers. In the literature only few papers have dealt with this problem, see for example [START_REF] Carrasco | Misspecied Structural Change, Threshold, and Markov-switching models[END_REF]. Thus, it appears very important to analyze the behavioral similarities created by these two models. Moreover, one should investigates and proposes a way to discriminate between them.

Generally, before applying a regime switching model, one should test the null hypothesis of a single state against the alternative of several states. Building such tests is problematic because of the presence of nuisance parameters which are not identied under the null hypothesis. For instance, these parameters are: the probabilities of transition p00 and p11 for the MSAR models and the threshold parameter r and the delay parameter d for the SETAR models. Moreover, in the case of the Markov switching models another problem occurs through the score function which is identically zero under the null. As a result, the Likelihood Ratio (LR), the Lagrange Multiplicateur (LM) and the Wald tests (W) have not their standard asymptotic distribution, see [START_REF] Davies | Hypothesis testing when a nuisance parameter is present only under the alternative[END_REF][START_REF] Davies | Hypothesis testing when a nuisance parameter is present only under the alternative[END_REF], [START_REF] Hansen | The likelihood ratio test under nonstandard conditions: testing the Markov Switching model of GNP[END_REF][START_REF] Hansen | Inference when a Nuisance Parameter is not Identied under the Null Hypothesis[END_REF], [START_REF] Gong | Testing Under Non-Standard Conditions in Frequency Domain: With Applications to Markov Regime Switching Models of Exchange Rates and Federal Funds Rate[END_REF], [START_REF] Garcia | Asymptotic null distribution of the likelihood ratio test in Markov switching models[END_REF][START_REF] Carrasco | Optimal Test for Markov Switching[END_REF].

This paper has two mains objectives. First, we analyze the behavior of the sup LR test against the particular alternative of a SETAR model. Precisely, we investigate the behavior of this test with respect to the delay parameter d and dierent values of the mean and the noise variance. Then, we applied the previous results to two real data sets, largely used in the empirical literature. The results show that the sup LR test rejects the null hypothesis of linearity whatever the switching models under the alternative (a SETAR or a MSAR models). This means that when the null hypothesis is rejected, it appears that dierent switching models are signicant . So, one should be careful to interpret this result because the model under the alternative is a possible candidate but other nonlinear models can be also candidates. Moreover, we show that discrimination between the two models is impossible as soon as the data set's sample size is large.

The outline of the paper is as follows. In Section 2, we discuss the sup LR test proposed by [START_REF] Garcia | Asymptotic null distribution of the likelihood ratio test in Markov switching models[END_REF] under the null hypothesis of an AR process and the alternative of a MSAR model. In Section 3, we report some new results concerning the possibility of dierentiation between the SETAR and MSAR models. In Section 4, we investigate the US/UK exchange rates series and the US GNP growth rate. Section 5 concludes. Hamiton (1989) applied a Markov switching model, with autoregressive order equal to four, to the growth rates of U.S GNP but [START_REF] Hansen | The likelihood ratio test under nonstandard conditions: testing the Markov Switching model of GNP[END_REF] and [START_REF] Garcia | Asymptotic null distribution of the likelihood ratio test in Markov switching models[END_REF] show, based on Sup LR test, that the null hypothesis of AR(4) cannot be rejected. [START_REF] Potter | A Nonlinear Approach to US GNP[END_REF] applied SETAR model to the U.S GNP but [START_REF] Hansen | Inference when a Nuisance Parameter is not Identied under the Null Hypothesis[END_REF] doubts about the evidence for the SETAR model.

2 Models and Testing procedure:

This section introduces the Markov switching model, the SETAR model and the sup LR test developed by [START_REF] Garcia | Asymptotic null distribution of the likelihood ratio test in Markov switching models[END_REF].

2.1

The Models

Consider the stationary Markov switching model (yt) dened by:

yt = φ1 + (φ2 -φ1)st + zt
(1)

with zt = θzt-1 + ut,
where u t is a Gaussian strong white noise N(0,σ 2 u ). We assume that the state st is independent of ut. The parameters φ1, φ2 and θ take values in ℜ. The state st is an unobserved Markov chain whose transition probability is dened by:

p(s t = j|s t-1 = i) = p ij , i, j = 0, 1, (2)
with 0 ≤ pij ≤ 1 and 2 1 j=0 pij = 1, i = 0, 1, and the transition matrix is

P = p 00 1 -p00 1 -p 11 p 11
.

We denote πi the unconditional probabilities for the process (st) to be in each regime : πi = P (st = i), i = 0, 1. These unconditional probabilities are equal to :

π0 = P (st = 0) = 1-p 11
2-p 00 -p 11 and π1 = P (st = 1) = 1-p 00 2-p 00 -p 11 .

In the Markov switching model (1) the 'state' or 'regime' plays an important role. Indeed, [START_REF] Hamilton | A new Approach to the Economic Analysis of Non stationarity Times Series and the Business Cycle[END_REF] suggests that the existence of discrete 'regimes' explain the nonlinearity in GNP growth rates. The rst 'state' will correspond to a fast growth and the second one to a slow growth. Here we assume that the MSAR model is strictly stationary and β-mixing. [START_REF] Guégan | Regime Switching Models: Real or Spurious Long Memory?[END_REF] show that a sucient condition for strict stationarity for Markov switching model is (1 -p00 -p11) log |θ| ≤ 0, see also [START_REF] Yao | On stability of nonlinear AR processes Markov switching[END_REF] for geometric ergodicity of MSAR Models.

We consider also the stationary rst-order threshold autoregressive process (yt) introduced by Lim et Tong (1980):

yt = φ1 + (φ2 -φ1)st + zt (3)
with zt = θzt-1 + ut, and

st = I{y t-d ≤ r}= 0 if y t-d ≤ r 1 if y t-d > r
where r and d are the threshold and delay parameters and ǫt is a Gaussian strong white noise N(0,σ 2 ǫ ). The necessary and sucient conditions for the geometrical ergodicity of (yt) in model ( 3) is simply θ ≤ 1, see [START_REF] Chen | On the Ergodicity of Tar(1) Process[END_REF]. In simulations, we assume that r = 0 and d = 1, 2.

The SETAR (3) and the MSAR (1) models are intimately related. In both models, changes in regimes is permanent contrary to structural change model where change occur only one time in the series. Moreover, the SETAR model is a special case of the Markov switching model when θ = 0, although in that case the Markov chain I{y t-d ≤ r}, d = 1, 2 is endogenous contrary to the Markov switching model proposed by [START_REF] Hamilton | A new Approach to the Economic Analysis of Non stationarity Times Series and the Business Cycle[END_REF] where the Markov chain is exogenous. Despite these similarities, the main dierence between the two models concerns the processus that govern changes in regimes under each model. Contrary to the MSAR models where changes in regimes are governed by an unobserved Markov chain, under the SETAR models changes in regimes occur when an observed variable y t passes a certain threshold parameter r.

Figures 1234, in appendix, give some representations of the trajectories of the MSAR and SETAR models. These gures show that the trajectories are similar and one cannot dierentiate between them. A same conclusion can be drawn from the scatters plots of yt versus y t-1. So, this graphical analysis can lead to misspecication which means that if researchers based their choice only in simple linear properties, they will be misleading and a misspecied model will be chosen. This create a problems concerning predictions and forecasting. Thus, it is very important to investigate the possibility of discrimination between dierent switching regressions models.

2.2

The Sup LR test for Markov

Switching Model

The literature on testing, when nuisance parameters are present under the alternative hypothesis, has growing rapidly and a variety of statistical tests have been developed. Most of the tests adopt the approach developed by [START_REF] Davies | Hypothesis testing when a nuisance parameter is present only under the alternative[END_REF][START_REF] Davies | Hypothesis testing when a nuisance parameter is present only under the alternative[END_REF], which proposes a sup LR test. The weakness of this test lies on the fact that we do not know the asymptotic distribution of this test under the alternative. [START_REF] Hansen | The likelihood ratio test under nonstandard conditions: testing the Markov Switching model of GNP[END_REF] proposes the likelihood ratio statistic. He considers the likelihood function as a function of unknown parameters, and he gets a bound for the asymptotic distribution of the test. The Hansen's approach (1992) is time consuming and makes it inappropriate in applications. In another hand, this test provides only a bounds and in terms of decision theory, it does not provide critical values. [START_REF] Andrews | Test for Parameter Instability and Structural Change with Unknown Change Point[END_REF] and [START_REF] Andrews | Optimal test when a nuisance parameter is Present only under the alternative[END_REF] proposed the sup LM test and a class of average exponential LM, Wald and LR tests. They showed that they are optimal in terms of weighted average power. [START_REF] Andrews | Admissibility of th likelihood Ratio Test when a Nuisance Parameter is Present Only under The Alternative[END_REF] show that the sup LR test is asymptotically admissible. It is the best test against alternatives that are suciently distant from the null hypothesis. [START_REF] Gong | Testing Under Non-Standard Conditions in Frequency Domain: With Applications to Markov Regime Switching Models of Exchange Rates and Federal Funds Rate[END_REF] developed two statistic tests for Markov switching models: the Dierence Test DT N (analogous to the LR test) and the LM test in the frequency domain. They derive their exact asymptotic null distributions under the condition of unidentied nuisance parameters. They show that, they only have to face to the problem of unidentied nuisance parameters in nonlinear context because the singularity problem disappears. Recently, [START_REF] Carrasco | Optimal Test for Markov Switching[END_REF] proposed the sup TS test which is 'asymptotically equivalent to Garcia's test in the sense that both are some kind of likelihood ratio tests and hence they are expected to have similar powers'.

As shown in the literature the likelihood ratio statistics behave relatively smoothly when a nuisance parameter is present under the alternative contrary to the Wald tests. The sup LR test is the most widely used and suggested to be the best test under such irregularity.

Let be the model (1). We test,

H0 : φ1 = φ2 against the alternative H1 : φ1 = φ2. (4)
This means that we test a linear AR model under (H0) against a MSAR model under (H1). Following the works of [START_REF] Davies | Hypothesis testing when a nuisance parameter is present only under the alternative[END_REF][START_REF] Davies | Hypothesis testing when a nuisance parameter is present only under the alternative[END_REF], [START_REF] Hansen | The likelihood ratio test under nonstandard conditions: testing the Markov Switching model of GNP[END_REF][START_REF] Hansen | Inference when a Nuisance Parameter is not Identied under the Null Hypothesis[END_REF] and [START_REF] Garcia | Asymptotic null distribution of the likelihood ratio test in Markov switching models[END_REF], we use the statistic:

LR = 2[L( β, γ) -L( β)],
(5) where L(.) is the log-likelihood function, β = (φ1, φ2, θ, σ 2 u ) and the vector of nuisance parameter is given by γ = (p 00 , p 11 ). For the statistic LR, β and γ is the maximum likelihood estimator of β under the alternative of Markov switching model, and β is the estimated value of β under the null hypothesis (H 0). [START_REF] Garcia | Asymptotic null distribution of the likelihood ratio test in Markov switching models[END_REF] use the distributional theory of [START_REF] Davies | Hypothesis testing when a nuisance parameter is present only under the alternative[END_REF][START_REF] Davies | Hypothesis testing when a nuisance parameter is present only under the alternative[END_REF], Hansen (1991a[START_REF] Hansen | Inference when a Nuisance Parameter is not Identied under the Null Hypothesis[END_REF] and [START_REF] Andrews | Optimal test when a nuisance parameter is Present only under the alternative[END_REF] in order to determine the asymptotic distribution theory of usual tests statistics. His method ! is based on the theorem showed by [START_REF] Hansen | Inference when a Nuisance Parameter is not Identied under the Null Hypothesis[END_REF]. This theorem shows that 'under certain regularity conditions, the asymptotic distribution of standard tests statistics are shown to be functionals of chi-square processes' . This method considers the LR test as the Supremum of a chi-square functional,

LR N = Supγ∈ΓLRN (γ) ⇒ SupC = Supγ∈ΓC(γ),
where Γ ∈]0, 1[ and C(γ) ! is a chi -square processus with covariance matrix K(., .) given by,

K(γ1, γ2) = R k V (γ1) -1 K(γ1, γ2)V (γ2) -1 R ′ k ,
where R k is a vector with ones in the position of the parameters constrained and zero elsewhere, k is the dimension of the parameter space under the alternative, and:

K(γ1, γ2) = limn→∞ E [S c N (β0, γ1)S c N (β0, γ2)] , S c N (β0, γ) = ∂ ∂β QN (β, γ), V (γ) = V (θ 0, γ).
To derive the asymptotic distribution of LR test under H 0, [START_REF] Garcia | Asymptotic null distribution of the likelihood ratio test in Markov switching models[END_REF] starts by deriving the covariance function of the chi-square processus C(γ). Then, the author proceeds as if the distributional theory applied and shows by Monte Carlo simulation that the LR asymptotic distributions give a good approximations of the empirical distributions. We use the critical values given by [START_REF] Garcia | Asymptotic null distribution of the likelihood ratio test in Markov switching models[END_REF]. They are equal to 10.89 for the MSAR with non-autoregressive order variable and 8.68 for the MSAR (1) model for the 5% signicance level.

To avoid the possibility of local maxima we use a lot of starting values in order to be sure that the maximum obtained is a global one.

Simulation Experiment

In this section, we investigate the size and the empirical power of the test introduced in (5) under the assumptions (H0) and (H1). This permits to calibrate the test and to specify its sensitivity to dierent values of the mean and noise's variance parameters. Then, using 1000 simulated series of the SETAR processes dened in (3), we assess the capability of Garcia's test to reject this last model. In all the empirical study we use the 5% signicant level.

see [START_REF] Hansen | Inference when a Nuisance Parameter is not Identied under the Null Hypothesis[END_REF] for the proof of this theorem and the conditions to be checked.

! see [START_REF] Garcia | Asymptotic null distribution of the likelihood ratio test in Markov switching models[END_REF] for the method for simulating Chisquare processes, Appendix.4, 786-7.

φ 2 = -φ 1 = 0.5
1 0.027 0.020 0.021 4 0.019 0.018 0.018 0.36 0.024 0.017 0.020

φ 2 = -φ 1 = 1
1 0.026 0.020 0.020 4 0.023 0.021 0.022 0.36 0.021 0.018 0.021

φ 2 = -φ 1 = 2
1 0.023 0.019 0.020 4 0.021 0.019 0.019 the size of the test ( 5) is not sensitive neither to the mean nor to the noise's variance. The test provides a size around 2% which means that the test underreject (H1), whatever the sample size. Now, to assess the power of the test we generate a two states Markov switching model (1) with θ = 0.5, p00 = 0.95, p11 = 0.95 and several means and noise's variances. The results are reported in table 2. For small samples, the power depends on the noise's variance. The empirical power is higher than 0.8 except in three cases. When we use a samples size greater than 300, the test has an empirical power close to 1. Table 2 shows also that the power of this test depends on the ratio,

|(φ 2 -φ1)| /σ 2 . (6)
For instance, if we take φ2 = -φ1 = 0.5 with σ = 1 or φ 2 = -φ 1 = 1 with σ = 2 and φ 2 = -φ 1 = 2 with σ = 0.6 we observe that when the value of the ratio ( 6) is large, the empirical power of the test ( 5) is close to 1 and small when the value of ( 6) is very small. An interesting case is when φ 2 = -φ1 = 0.5 and σ = 2 , for N=100 and N=300, the power is smaller. This result can be explained by the high volatility of the data compared to the small value of the mean. " This empirical study shows that the test ( 5) is able to recognize a Markov switching model even using small samples sizes. Nevertheless, we observe through the articles of [START_REF] Garcia | Asymptotic null distribution of the likelihood ratio test in Markov switching models[END_REF], [START_REF] Gong | Testing Under Non-Standard Conditions in Frequency Domain: With Applications to Markov Regime Switching Models of Exchange Rates and Federal Funds Rate[END_REF] and [START_REF] Coe | Powers Issues when testing the Markov switching model with the sup likelihood ratio test using U.S. output[END_REF], that using real data, the results are no so evident.

In a simulation study, not reported here, our results concerning the size and the empirical power of the sup LR test are close to those of [START_REF] Carrasco | Misspecied Structural Change, Threshold, and Markov-switching models[END_REF] when under (H0) we use a strong white noise.

3.2

Capability of the Sup LR test to reject SETAR model Now, to study the ability of the sup LR test to identify the alternative as a non Markov switching model when the DGP is a SETAR model, we simulate 1000 time series from model (3) with dierent samples sizes N=100, 200, 300, 400, 500 and 1000. We set the threshold parameter and the initial value y0 equal to zero.

Six combinations for the parameters (φ1, φ2) are considered. The values of the noise's variance are setted equal to σ = 0.6, 1 and 2. All combinations are chosen in order to be sure that we have enough points in each regime except for one combination parameters. To minimize the inuence of starting values we have discarded the rst 200 observations. We do not investigate the inuence of the autoregressive parameter θ, which is setted equal to 0.5. [START_REF] Garcia | Asymptotic null distribution of the likelihood ratio test in Markov switching models[END_REF] shows that the distribution of the test is not sensitive to the value of the autoregressive parameters.

In the following paragraph, we analyze the results reported in tables 3-6. These tables report the empirical power of the sup LR test, percentage of rejections of the null assumption, with respect to dierent values of the delay parameter d.

d=1 in (6)

1-Results for the SETAR(0) "

The results of simulation, when the DGP follows the SETAR process (3) with r = 0, d = 1 and θ = 0, are reported in table 3.

For a sample size larger than 200, the sup LR test correctly rejects the null hypothesis of linear model. Under the alternative, the sup LR test is unable to recognize this SETAR process. It accepts nearly always the SETAR process although we use a statistic built to recognize the MSAR process (1). Garcia (1998, appendix.3, p.785) shows that the LR test has the same asymptotic distribution under (H0) whatever the process that we consider under the alternative: MSAR(0) or SETAR(0). Our empirical results conrm their theoretical nding. # Table 4: Power of Sup LR test when the DGP is a SETAR (1) process with d=1

(φ 1 , φ 2 ) σ 2
100 200 300 400 500 1000 p 00 and p 11 0.36 22.4 36.6 50 55.6 60.4 64.2 p 00 = 0.79, p 11 = 0.70 (-0.5, 0.5) 1 5.5 10.1 11.9 12.2 15.5 21.4 p 00 = 0.80, p 11 = 0.65 4 2.3 3.7 4 4.9 5.7 6.7 p 00 = 0.82, p 11 = 0.54 0.36 96.1 100 100 100 100 100 p 00 = 0.85, p 11 = 0.85 (-1, 1) 1 33.4 66.6 84.7 90.6 94.5 100 p 00 = 0.73, p 11 = 0.73 4 6.2 9.3 14.7 16.3 21 25.8 p 00 = 0.79, p 11 = 0.50 0.36 99.9 100 100 100 100 100 p 00 = 0.92, p 11 = 0.93 (-1.5, 1.5) 1 87 98.5 100 100 100 100 p 00 = 0. Now, by analyzing the probabilities to remain in the same regime, we observe that p00 and p11 are equals when the means (φ1 and φ2) are symmetrical with respect to the threshold r. These value p00 and p11 increase with (φ1, φ2) and decrease with σ. Their behaviors depend on the expressions p 00 = Φ(r -φ1/σ) and p11 = Φ(r -φ2/σ), where Φ(.) is the c.d.f (cumulative distributive function) of the standard Gaussian distribution, see Appendix for more details. When φ2 = -φ1 = 2 and σ = 0.6, we get a very small empirical power for the sup LR test. In this later case, among the 1000 generated series only one hundred series present changes from one regime to another one # . This is due to the high value of the mean and the small value of the noise's variance. For these series, p00 = 0.999 which indicates that the probability to change from one regime to another one is very small.

2-Results for SETAR(1)

Table 4 gives the results of the empirical power of the sup LR test for dierent sample sizes when the data are generated under model (3) using d = 1, r = 0 and θ = 0.5. For large sample sizes, the test has no ability to dierentiate between SETAR(1) and MSAR(1) models. We analyze below in more details the behavior of this test under the alternative.

• Assume that σ is xed: When the value of φ2 and φ1 are large, φ2 = -φ1 = 1.5 or φ2 = -φ1 = 2, the sup LR test rejects the null with high empirical power. This means that if the data are generated from SETAR(1) this test builded to recognize a MSAR(1) model accepts, with a very high probability, the SETAR(1) as a MSAR(1) model.

Again, we observe that the test cannot discriminate between the two models, in particular when the data are less noisy (σ = 0.6 or σ = 1). In another hand, when the dierence between the two means decreases, for instance when φ1 and φ2 are small, the test can distinguish between the two models ( 1) and ( 3). The empirical power to reject the SETAR(1), for φ2 = -φ1 = 1.5 and φ 2 = -φ1 = 2 with σ = 2 is equal to 83.4% and 65.3% respectively, using N=100 observations.

• Assume that φ1 and φ2 are xed: By varying the noise variance σ, we observe that the ability of the sup LR test to discriminate between the two models (1) and ( 3) increases specially when the data are very noisy. For example, when φ 1 = -0.8, φ2 = 0.5 and N=100, the rejection of the alternative of SETAR(1) model increases from 65% (σ = 0.6) to 88.1% (σ = 1) and 96.4% (σ = 2). This means that when the noise variance σ increases the data set becomes very noisy and p 00 + p11 decreases. Thus, the switches between the two states increase and the test rejects the alternative in most of the cases. For example, for N=100 with φ1 = -0.8, φ2 = 0.5, we observe that p00 + p11 decreases from 1.56 when (σ = 0.6) to 1.5 when (σ = 1) and then to 1.39 when (σ = 2).

d=2 in (6)

Table 5 reports the empirical powers of the sup LR test when the Data Generated Process is a SETAR(0) model with d = 2, r = 0 and θ = 0 in (6).

The results show that when the sample size is large (N=1000) the test has a reasonable empirical power against the null hypothesis of a linear model, except for a few combinations. When the value of the mean $ increases, the power of the test increases too. In these latter cases, the ability of the sup LR test to discriminate between the MSAR(0) and the SETAR(0) models is larger in small samples, specially when the data set is very noisy. Two series seem to have a specic behaviors, when φ 2 = -φ1 = 0.5 with σ = 2 and φ2 = -φ1 = 2 with σ = 0.6. For these two processes the empirical power is very low and the ratio (6) attains respectively its minimum and its maximum values. From the empirical results, it appears that when the ratio (6) is very high or very small, the power of the test is high and small in the other cases.

Table 6 provides the empirical power of the sup LR test when the data are generated with a SETAR(1) model using d = 2 and θ = 0.5. In this case, the test has a good empirical power against the null hypothesis of a linear model like in previous cases, even if the sample size is small. This means that the test is unable to dierentiate the SETAR model from the MSAR one. From table 6, it appears that for a small sample size, N=100 observations, the power is not sensitive neither to the mean nor to the noise's variance. The empirical power is around 60%, except for the two series that have the lower and higher values of the ratio |(φ2 -φ1)| /σ 2 . In these two cases, the power is 16% and 47.7% respectively.

After analyzing the behavior of the sup LR test, it is interesting to assess its behavior when we consider a real data. From the economic and nancial time series, we are interesting by the series of the US GNP growth and the US/UK exchange rate. For each one, we applies the results obtained from the previous empirical simulations to investigate which model (SETAR or MSAR) describes better their evolutions.

Applications

In economic and nancial domains, two particular series have been widely examined by researchers to justify the existence of shifts and change between regimes: the US GNP growth rate and the US/UK exchange rate. Many factors induce nonlinearities inside those time series. For the US GNP growth rate, many researchers argue that this variable is characterized by an asymmetric behavior and suggest that shocks are more persistent during great depression. In the other hand, the presence of nonlinearities and the adoption of switching model, for the US/UK exchange rates, is motivated mainly by the heterogeneity of the participants in the foreign exchanges markets, transaction costs, and the dierences between domestic and foreign monetary and scal policies. Also, the rigidities of certain markets and the presence of chartists and fundamentalists in the foreign exchanges markets induce dierences in opinions and in expectations.

For the rst time series, [START_REF] Hamilton | Analysis of Time Series Subject to Change in Regime[END_REF] proposes a MSAR(4) process to model the growth rate of the US GNP. However, [START_REF] Hansen | The likelihood ratio test under nonstandard conditions: testing the Markov Switching model of GNP[END_REF] doubts in Hamilton specication and proposes a constrained model in which he allows to the intercept, the slope parameters, and the error variance to shift between the 'states'. [START_REF] Garcia | Asymptotic null distribution of the likelihood ratio test in Markov switching models[END_REF] conrms that there is no evidence for a MSAR(4) model for the US GNP growth rate. In another hand, this same series have been modeled by the SETAR model. For instance, [START_REF] Potter | A Nonlinear Approach to US GNP[END_REF] proposes a SETAR(5) model without the third and fourth lags. [START_REF] Hansen | Inference when a Nuisance Parameter is not Identied under the Null Hypothesis[END_REF] doubts that the SETAR model proposed by [START_REF] Potter | A Nonlinear Approach to US GNP[END_REF] captures changes in the US GNP growth rate series. % For the US/UK exchange rate, [START_REF] Engle | Long Swings in the Dollar: Are They in the Data and Do Markets Know It?[END_REF] propose a MSAR model without autoregressive order. Under this specication, the US/UK exchange rate follows a random walk specication under the null and the alternative hypothesis, see also [START_REF] Cheung | Exchanges Rates and Markov Switching Dynamics[END_REF].Now, we propose to use the previous test (5) to test the presence of changes in regimes inside these time series. If the test is positive $ , we investigate the two alternatives of a MSAR or a SETAR model using the previous results given in simulations.

The data

We use a quarterly data set for the U.S real GNP, over the period January 1947 to April 2005. This provides us a sample size of 232 points. For the US/UK exchange rates, we use two frequency data sets, monthly and quarterly data. In the rst case we have 176 points, from January 1986 to September 2000, and 58 points in the latter case from January 1986 to October 2000.

The GNP data set is provided from the website of Federal Reserve Bank at St Louis in USA and the exchange rate data set comes from Data stream Base. In order to make the data stationary we use the following transformation 100 * [log(Xt) -log(Xt-1)], where Xt represents the observed data.

Results

First, we start by estimating a SETAR process for the GNP growth rate and the US/UK exchange rate data.

The results are provided in table 7, using d=1 in model ( 6). Here we are interested by the value of the ratio given in the last line of the table 7. In all cases, this value is very small. Then, following the results given in paragraph 3, the sup LR test rejects the SETAR model and accepts the null hypothesis of a linear model.

To conrm this result, we simulate 1000 series, using the previous estimated parameters for the SETAR processes, with N=100, 200, 300, 400 and 500 samples sizes. Then, we compute the expected empirical power of the sup LR test for each experiment. The results are given in table 8. In all cases, we reject the alternative with a high power. This means that the SETAR(1) specication is inappropriate. Thus, we adjust a Markov switching process for these data sets. In table 9, columns 2 and 3, we provide the estimation of the real growth rate GNP data using an AR(1) linear model and a MSAR(1) model. We provide also In the table 9, colums 4-7, we provide an estimate of the monthly and quarterly US/UK exchange rate data. Under the null, the series are modeled by a random walk and by a Markov switching model under the alternative. The likelihood ratio statistic for the monthly data is equal to 19.38. This value is largely greater than the asymptotic critical value, 10.89. Thus, for this US/UK series we accept a Markov switching model against a random walk process. Using quarterly data (58 observations) the likelihood ratio statistic is equal to 6.9. This value does not excess the Garcia's 95% asymptotic critical value of 10.89. Thus, for the quarterly data, we accept the random walk specication (linear model) against the Markov switching model. We are not surprised by this result because the sample size and the frequency of the data plays an important role for the choice of the best model. Now, we propose a simulation using the estimated parameters of the MSAR(0) for US/UK exchange rate given in table 7. In order to study the inuence of the sample size on the empirical power of the Sup LR test, we simulate 1000 series for dierent samples sizes, N=100, 200, 300, 400, 500.

The simulations results show that the power of the test is equal to 85% when N=100, and close to 1 when the sample size is N ≥ 200. We remark also that there is small dierence between the empirical power for the monthly and the quarterly data set. This comes from the values obtained for the ratio (6).

In ne, we retain a MSAR(1) model for the US GNP growth rate, a MSAR(0) model for the US/UK monthly data and a random walk when we use a quarterly data set for the US/UK.

Conclusion

This paper study the power of sup LR test (5), constructed for a MSAR model, when the underlying simulated process is a SETAR model. We explore the sensitivity of the empirical power with respect to the mean, the variance and the delay parameter d of the model (6). The results show that it is very dicult to discriminate between the SETAR model and the MSAR model, in particular using large sample sizes. For small sample sizes the Sup LR test allows to doubt about the nature of the changes in regimes. It seems that the higher and the lower value of the ratio |(φ 2 -φ1)| /σ 2 reduce the power of the sup LR test for small samples, but in large samples this test still has good power.

Tow specic cases are of interest, when the ratio is very lower and very greater. For these two extreme cases the test has a correct power to discriminate between dierent types of switching models. We apply this approach to the US GNP growth rate and the US/UK exchange rate and we show that a Markov switching specication is more appropriate for the US GNP growth rate and for the US/UK exchange rate (Monthly data). For the US/UK Quarterly data we accept a random walk specication because we have only a few observations, 58 points.

'

Figure 2 :

 2 Figure 2: (C) Trajectory of the MSAR model when p 00 = 0.98, p 11 = 0.98, µ 0 = -µ 1 = 2 and φ 1 = 0.5, and (D) trajectory of the SETAR model when r = 0, d = 1, µ 0 = -µ 1 = 2 and φ 1 = 0.5.

Figure 3 :

 3 Figure 3: (E) Trajectory of the MSAR model when p 00 = 0.95, p 11 = 0.95, µ 0 = -µ 1 = 2 and φ 1 = 0.5, and (F) trajectory of the SETAR model when r = 0, d = 2, µ 0 = -µ 1 = 2 and φ 1 = 0.5.

Figure 4 :

 4 Figure4: Scatterplots of, (A) MSAR model when p 00 = 0.98, p 11 = 0.98, µ 0 = -µ 1 = 2 and φ 1 = 0, (B) SETAR model when r = 0, d = 1, µ 0 = -µ 1 = 2 and φ 1 = 0, (C) MSAR model when p 00 = 0.98, p 11 = 0.98, µ 0 = -µ 1 = 2 and φ 1 = 0.5, (D) SETAR model when r = 0, d = 1, µ 0 = -µ 1 = 2 and φ 1 = 0.5, (E) MSAR model when p 00 = 0.95, p 11 = 0.95, µ 0 = -µ 1 = 2 and φ 1 = 0.5 and nally (F) when SETAR model r = 0, d = 2, µ 0 = -µ 1 = 2 and φ 1 = 0.5.

Table 3 :

 3 Power of sup LR test when the DGP is SETAR (0) with d=1

			(φ 1 , φ 2 )	σ 2		100	200	300	400	500	1000	p 00 and p 11
					0.36		99.3	100	100	100	100	100	p 00 = 0.86, p 11 = 0.86
			(-0.5, 0.5)	1		74.3	97.5	100	100	100	100	p 00 = 0.80, p 11 = 0.80
					4		13.8	38.5	59.1	74.6	99.6	100	p 00 = 0.74, p 11 = 0.72
					0.36		100	100	100	100	100	100	p 00 = 0.95, p 11 = 0.95
			(-1, 1)	1		98.3	100	100	100	100	100	p 00 = 0.88,p 11 = 0.88
					4		66.8	97.7	100	100	100	100	p 00 = 0.80, p 11 = 0.80
					0.36		45.5	69.2	82	90.5	94.5	100	p 00 = 0.99, p 11 = 0.99
			(-1.5, 1.5)	1		99.4	100	100	100	100	100	p 00 = 0.94, p 11 = 0.94
					4		97.8	100	100	100	100	100	p 00 = 0.84, p 11 = 0.84
					0.36		3.4	9	10.2	15.7	18.4	26.4	p 00 = 0.999, p 11 = 0.77
			(-2, 2)	1		88.7	98.5	100	100	100	100	p 00 = 0.98, p 11 = 0.97
					4		34.7	64.3	84.7	93.8	96.3	100	p 00 = 0.88, p 11 = 0.88
					0.36		99.5	100	100	100	100	100	p 00 = 0.93, p 11 = 0.85
			(-0.8, 0.5)	1		89.4	100	100	100	100	100	p 00 = 0.89, p 11 = 0.84
					4		29.4	65.5	88.6	96.5	99.3	100	p 00 = 0.88, p 11 = 0.84
					0.36		13.8	32.4	43.3	54	63.2	82.5	p 00 = 0.68, p 11 = 0.95
			(-1.5, 0)	1		52.6	83.5	96.8	99.3	99.7	100	p 00 = 0.69, p 11 = 0.94
					4		28.4	68	89.6	96.5	99.6	100	p 00 = 0.70, p 11 = 0.82
	Table	2:	Empirical	power	of	the	sup	LR
	test	when	the	DGP	is	a	MSAR(1)	process
	(φ 1 , φ 2 )	σ 2	100	300		1000
			0.36	97	100		100
	(-0.5, 0.5)	1	79.3	100		100
			4	15.5	60.1		99.3
			0.36	98.4	100		100
	(-1, 1)	1	98.3	100		100
			4	78	100		100
			0.36	87.6	98.2		100
	(-2, 2)	1	95.2	99.8		100
			4	99	100		100

Table 5 :

 5 Power of sup LR test when the DGP is a SETAR (0) model with d = 2 and θ = 0.

	(φ 1 , φ 2 )	σ 2	100	200	300	400	500	1000	p 00 and p 11
		0.36	36.3	72.1	87.9	96.7	99.2	100	p 00 = 0.94, p 11 = 0.94
	(-0.5, 0.5)	1	14.3	34.3	50.8	67.7	80.4	99	p 00 = 0.91, p 11 = 0.91
		4	4.1	6.7	11.1	17.4	24.8	53.1	p 00 = 0.88, p 11 = 0.64
		0.36	59.7	90.9	97.7	98.4	99.1	100	p 00 = 0.94, p 11 = 0.94
	(-1, 1)	1	45.4	79.7	93.4	98.7	99.7	100	p 00 = 0.97, p 11 = 0.96
		4	13.3	34.3	55.2	69.6	79.1	99	p 00 = 0.90, p 11 = 0.90
		0.36	28.7	51	64.6	74.3	92.5	100	p 00 = 0.73, p 11 = 0.57
	(-1.5, 1.5)	1	57.7	86	94	98.8	99.5	100	p 00 = 0.97, p 11 = 0.96
		4	34	64.3	83.2	91.6	97.1	99.6	p 00 = 0.93, p 11 = 0.93
		0.36	3.7	8.2	12.8	23.3	29	38.2	p 00 = 0.69, p 11 = 0.85
	(-2, 2)	1	57.7	87	97.5	100	100	100	p 00 = 0.64, p 11 = 0.53
		4	45.6	79.9	93	98.1	100	100	p 00 = 0.95, p 11 = 0.91
		0.36	44.4	79.3	95.5	98.1	99.9	100	p 00 = 0.93, p 11 = 0.95
	(-0.8, 0.5)	1	20.8	52.7	73.3	87.1	96.2	100	p 00 = 0.90, p 11 = 0.93
		4	5.8	14.6	22.2	30.6	44.2	78	p 00 = 0.89, p 11 = 0.88
		0.36	5.5	12.3	23.4	29.7	30.8	54.6	p 00 = 0.73, p 11 = 0.98
	(-1.5, 0)	1	10.6	32.5	47.4	63.8	75.3	99	p 00 = 0.84, p 11 = 0.96
		4	12.6	14	27.3	36.6	43.9	82.1	p 00 = 0.86, p 11 = 0.90

Table 6 :

 6 Power of sup LR test when the DGP is SETAR (1) with d = 2 and θ = 0.5.

	(φ 1 , φ 2 )	σ 2	100	200	300	400	500	1000	p 00 and p 11
		0.36	73.4	96.7	99.5	100	100	100	p 00 = 0.60, p 11 = 0.60
	(-0.5, 0.5)	1	50.1	81	96.1	98.6	99.9	100	p 00 = 0.67, p 11 = 0.67
		4	16	36.4	51.9	68.5	79.9	98.2	p 00 = 0.66, p 11 = 0.64
		0.36	66.8	92.5	97.8	98.1	99.4	100	p 00 = 0.65, p 11 = 0.47
	(-1, 1)	1	74.4	95.5	99.5	99.9	100	100	p 00 = 0.55, p 11 = 0.55
		4	50	83.9	94.6	99.8	99.9	100	p 00 = 0.66, p 11 = 0.6
		0.36	66.3	90.7	95.2	98	99.2	100	p 00 = 0.39, p 11 = 0.37
	(-1.5, 1.5)	1	66.6	94	99.1	99.6	99.8	100	p 00 = 0.49, p 11 = 0.46
		4	66.9	96.1	99.4	99.8	99.9	100	p 00 = 0.61, p 11 = 0.61
		0.36	47.7	77.6	93.3	97.9	100	100	p 00 = 0.39, p 11 = 0.37
	(-2, 2)	1	65.3	92.1	96.5	99.5	100	100	p 00 = 0.43, p 11 = 0.40
		4	73	96.8	99.7	100	100	100	p 00 = 0.57, p 11 = 0.57
		0.36	74.7	97	98.2	99.9	100	100	p 00 = 0.62, p 11 = 0.55
	(-0.8, 0.5)	1	63.4	93	99.3	99.8	100	100	p 00 = 0.68, p 11 = 0.66
		4	29.2	53.8	74.3	86.4	95	100	p 00 = 0.66, p 11 = 0.65
		0.36	76.7	99.9	100	100	100	100	p 00 = 0.94, p 11 = 0.88
	(-1.5, 0)	1	71.5	98.1	99.7	100	100	100	p 00 = 0.76, p 11 = 0.70
		4	33.1	65.6	81.2	93	97.2	100	p 00 = 0.68, p 11 = 0.66

Table 7 :

 7 Estimates of SETAR model of the U.S GNP growth rate (Quarterly data) and the US/UK exchange rate (Monthly and Quarterly data)

	Parameter	U.S GN P	U S/U K(M ) U S/U K(Q)
		SETAR(1)	SETAR(0)	SETAR(0)
	φ 1	0.614	-0.551	0.044
		(0.1046)	(0.362)	(0.801)
	φ 2	1.172	0.446	-1.384
		(0.119)	(0.300)	(2.163)
	θ	0.598	-	-
		(0.049)
	σ 2	0.931	9.447	33.607
		(0.92)	(0.082)	(0.973)
	r	0.740	0.049	3.953
	d	1	1	1
	|(φ 2 -φ 1 )| /σ 2	0.599	0.105	0.042

Table 8 :

 8 Power of the Sup LR test when the DGP is the estimates parameter of real data (U.S GN P , U S/U K(M )

		and U S/U K(Q))
		100	200	300	400	500
	U.S GN P	3.4	3.6	4.5	5.4	6.4
	U S/U K(M )	6.3	14.4	22.3	35.3	44.7
	U S/U K(Q))	1.8	3.4	4.3	7.4	8.6

Table 9 :

 9 Estimates of MSAR model for the U.S GNP growth rate (Quarterly data) and the US/UK exchange rate (Monthly and Quarterly data)

	Parameter	U.S GN P	U S/U K(M )	U S/U K(Q)
		AR(1)	MSAR(1)	AR(0)	MSAR(0)	AR(0)	MSAR(0)
	φ 1	0.563	0.952	-0.018	0.320	-0.0743	0.766
		(0.095)	(0.103)	(0.231)	(0.228)	(0.739)	(0.863)
	φ 2	-	-1.213	-	-8.295	-	-13.855
			(0.467)		(1.536)		(5.583)
	θ	0.332	0.391	-	-	-	-
		(0.073)	(0.071)
	p 00	-	0.9618	-	0.967	-	0.950
			(0.023)		(0.020)		(0.048)
	p 11	-	0.262	-	0.205	-	0.227
			(0.223)		(0.181)		(0.425)
	σ 2 u	0.938	0.451	9.42	6.687	31.764	20.813
		(0.092)	(0.083)	(0.054)	(0.214)	(1.231)	(0.973)
	LL	-101.205	-96.736	-284.002	-274.483	-129.267	-125.823
	the value of the log-likelihood function under each
	model. In the case of the US GNP series, the value
	of the likelihood ratio statistic is 8.938. This value
	is greater than Garcia's 95% asymptotic critical value
	of 8.68. Thus, we accept the alternative of MSAR(1)
	model against the null of linear AR(1) model. The value
	of the ratio in (7) is between 1 and 10 for this series.
	This means that, if we use the estimate value of the U.S
	real GNP to generate articial data under an MSAR(1)
	model, the empirical power of the sup LR test will be
	close to 1. Simulation experiments results conrm this
	intuition. Their power is equal to 80.2%, 99.8% and
	100% for a sample size equal to N=100, N=200 and
	N=300 respectively, when the DGP use the estimated
	parameters for the U.S real GNP provided in table 9.
	These results induce to retain a MSAR(1) model for
	the U.S real GNP data.

Appendix

Consider the simple rst-autoregressive SETAR model:

with zt = θzt-1 + ut, and

This model can be rewriting as:

Therefore for the SETAR(1) the probability P r(st = 0|st-1, st-2, Y ) depends on yt-2 then the process I{y t-d ≤ r} is not an a exogenous Markov chain like an [START_REF] Hamilton | A new Approach to the Economic Analysis of Non stationarity Times Series and the Business Cycle[END_REF].

• When θ = 0, we see that the model ( 7) becomes a particular case of Markov Switching model because P r(st = 0|st-1, Y ) = P r(st = 0|st-1). For model ( 7) we get:

# .

• When θ = 0 and φ1 = -φ2 in (7), we obtain: