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Temporally Local Maximum Likelihood with Application to SIS Model

  • Christian Gourieroux und Joann Jasiak ORCID logo EMAIL logo
Veröffentlicht/Copyright: 7. März 2023
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Abstract

The parametric estimators applied by rolling are commonly used for the analysis of time series with nonlinear patterns, including time varying parameters and local trends. This paper examines the properties of rolling estimators in the class of temporally local maximum likelihood (TLML) estimators. We consider the TLML estimators of (a) constant parameters, (b) stochastic, stationary parameters and (c) parameters with the ultra-long run (ULR) dynamics bridging the gap between the constant and stochastic parameters. We show that the weights used in the TLML estimators have a strong impact on the inference. For illustration, we provide a simulation study of the epidemiological susceptible–infected–susceptible (SIS) model, which explores the finite sample performance of TLML estimators of a time varying contagion parameter.

JEL Classification: C01; C13; C22

Corresponding author: Joann Jasiak, York University, Toronto, Canada, E-mail:

  1. Research funding: The first author acknowledges financial support from the ACPR chair “Regulation and Systemic Risk”, the ERC DYSMOIA and the Agence Nationale de la Recherche (ANR-COVID) [Grant ANR-17-EURE-0010]. The second author gratefully acknowledges financial support of the Natural Sciences and Engineering Council of Canada (NSERC).

  2. Conflict of interest statement: The authors declare no conflicts of interest regarding this article.

  3. Author contribution: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.

Appendix A: Second-Order Expansion

Even though the TLML estimator is consistent, the finite sample bias adjustment formula is expected to be non-standard. We consider below the second-order expansion of the FOC [see Gosh and Subramanyan (1974), Efron (1975), Firth (1993), Kosmidis and Firth (2009)]. For expository purpose, we assume dim θ t = 1. We define:

L 1 t θ * = log l θ y t | y t 1 ; θ * , L 2 t θ * = 2 log l θ 2 y t | y t 1 ; θ * , L 3 t θ * = 3 log l θ 3 y t | y t 1 ; θ * .

The second-order expansion of the FOC in a neighbourhood of the pseudo-true value is:

h = 0 T 1 w ( h ) L 1 , T h θ * + h = 0 T 1 w ( h ) L 2 , T h θ * ( θ ̂ T ( w ) θ * ) + 1 2 h = 0 T 1 w ( h ) L 3 , T h θ * ( θ ̂ T ( w ) θ * ) 2 = o P ,

where o p is negligible in probability with respect to the left hand side of the equation, or equivalently,

(A.1) h = 0 T 1 w ( h ) L 1 , T h θ * + W T E 0 L 2 , t θ * ( θ ̂ T ( w ) θ * ) + h = 0 T 1 w ( h ) L 2 , T h θ * E 0 L 2 , t θ * ( θ ̂ T ( w ) θ ̂ * ) + 1 2 W T E 0 L 3 , t θ * ( θ ̂ T ( w ) θ * ) 2 ) = o P .

From (3.9), it follows that:

θ ̂ T ( w ) θ * = I T w ; θ * 1 / 2 W T J θ * X T + o P ,

where X T N(0, 1).

This expression can be plugged into the two last terms of the second-order expansion to get:

h = 0 T 1 w ( h ) L 1 , T h θ * = W T J θ * ( θ ̂ T ( w ) θ * ) + h = 0 T 1 w ( h ) L 2 , T h θ * + J θ * I T w ; θ * 1 / 2 W T J θ * X T + 1 2 W T E 0 L 3 , t θ * I T w ; θ * W T 2 J θ * 2 X T 2 = o P .

Let us consider the variable:

Z T = h = 0 T 1 k = 0 T 1 w ( h ) w ( k ) I 2 h k ; θ * 1 / 2 h = 0 T 1 w ( h ) 2 log l θ 2 y T h | y T h 1 ; θ * I 2 , T w ; θ * 1 / 2 h = 0 T 1 ( w ( h ) 2 log l θ 2 y T h | y T h 1 ; θ * ] ,

where I 2 h ; θ * = Cov 0 2 log l θ 2 y t | y t 1 ; θ * , log l θ 2 y t h | y t h 1 ; θ * .

Then we get:

θ ̂ T ( w ) θ * = I T w ; θ * 1 / 2 W T J θ * X T I T w ; θ * 1 / 2 W T J θ * I 2 T w ; θ * 1 / 2 W T J θ * X T Z T 1 2 E 0 L 3 , t θ * J θ * I T w ; θ * W T 2 J θ * 2 X T 2 + o P .

This expansion provides an approximation of the difference between the TLML estimator and the pseudo-true value as a quadratic function of the pair (X T , Z T ), which is asymptotically normally distributed and has zero mean components with unitary variances and non-zero correlation, in general. Alternative bias adjustments could be based on the pseudo score [Lambrecht et al. (1997)].

Appendix B: Stationarity Condition

B.1 Condition for Ergodicity

Whenever the Markov chain is irreducible, we can use the sufficient conditions for ergodicity provided in Tweedie (1975). They are:

  1. E [ p ̂ 2 ( t ) | p ̂ 2 ( t 1 ) ] is bounded.

  2. E [ p ̂ 2 ( t ) p ̂ 2 ( t 1 ) | p ̂ 2 ( t 1 ) = x ] ε , for x outside a compact set.

Condition (i) is satisfied since p ̂ 2 ( t ) < 1 .

Let us now consider condition (ii). We have:

E [ p ̂ 2 ( t ) p ̂ 2 ( t 1 ) | p ̂ 2 ( t 1 ) = x ] = a x ( 1 x ) c x < c x .

Therefore condition (ii) is satisfied with the compact set K = [ɛ/c, 1], and ɛ < c.

B.2 Condition for Irreducibility

The different types of behaviour discussed below Proposition 6 are related to the irreducibility properties of the Markov chain [see e.g. Rio (2017), Chapter 9, and Chotard and Auger (2019)]. This irreducibility follows from the condition 0 < 1 − c/a < 1, and replacing of B by B + (or P by P + ) in the definition of the chain.

Appendix C: Additional Figures

C.1 Quasi-Collinearity

To provide some insights on the quasi-collinearity, we compute the sample information matrix from the Hessian of the temporally local log-likelihood function and report its eigenvalues. As expected these eigenvalues are positive and one of them is small and close to 0.

Figure 12: 
Eigenvalues of estimated information matrix: Constant a.
Figure 12:

Eigenvalues of estimated information matrix: Constant a.

Figure 13: 
Eigenvalues of estimated information matrix: Stochastic a.
Figure 13:

Eigenvalues of estimated information matrix: Stochastic a.

C.2 Persistence of Estimates and Stochastic Parameters

Additional summary statistics provided below concern the joint dynamics of the stochastic parameters of interest and their local estimates. They are displayed in Figures 14 and 15 for the contagion parameter and reproductive number, respectively. For each value of w considered, the diagonal panels show the autocorrelations of the estimates and of the stochastic parameters, respectively. The off-diagonal panels show the cross-correlations between the estimates and stochastic parameters.

Figure 14: 
Joint ACF of a
t
 and 






a

̂



t



${\hat{a}}_{t}$


.
Figure 14:

Joint ACF of a t and a ̂ t .

Figure 15: 
Joint ACF of R0,t and 






R

̂



0
,
t



${\hat{R}}_{0,t}$


.
Figure 15:

Joint ACF of R0,t and R ̂ 0 , t .

The autocorrelation function (ACF) of a t and R0,t reveals the local-to-unity property of the ULR process. For w = 0.1, the weighted estimator is more global and appears almost uncorrelated with the underlying stochastic parameter. We observe that the cross-correlations, which often take small values, do not decay to zero with the lag. This is a consequence of the ULR a t process. For larger w, the estimates display more persistence, although the autocorrelations of a ̂ t are smaller and decay faster than those of a t .

C.3 Nonlinear Prediction Performance

Under the standard maximum likelihood approach, the values of the log-likelihood at the optimum can be used to perform tests based on the likelihood ratios, especially of the time varying hypotheses H0T = {R0T > 1}. It can be used to check the “joint” accuracy of a ̂ t , c ̂ t , as a global prediction measure. In this respect, Figure 16 displays the evolution of the locally weighted log-likelihood.

Figure 16: 
Log-likelihoods, stochastic a, red dashed line: w = 0.1, black solid line: w = 0.5 and dotted green line: w = 0.9.
Figure 16:

Log-likelihoods, stochastic a, red dashed line: w = 0.1, black solid line: w = 0.5 and dotted green line: w = 0.9.

This evolution has to be compared with the trajectory of counts in Figure 6. As expected, the local nonlinear fit is improved during the episodes of counts evolving without sudden jumps or “local” trends, i.e. characterized by rather stable evolution. In contrast, the values of the log-likelihood are shown to decrease in the neighbourhoods of peaks or troughs.

Alternative measures of prediction performance could also be built by comparing at each date the observed number of infected individuals N2(t) with its estimator-based prediction:

N ̂ 2 ( t ) = a ̂ t [ n N 2 ( t 1 ) ] N 2 ( t 1 ) n + ( 1 c ̂ t ) N 2 ( t 1 ) .

The difference N 2 ( t ) N ̂ 2 ( t ) depends on the theoretical prediction error (which is present when the true parameter values are used) and the estimation error due to replacing the true parameters by their estimates in the theoretical pointwise prediction formula.

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Received: 2022-05-30
Revised: 2022-10-31
Accepted: 2023-02-07
Published Online: 2023-03-07

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