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488—Chapter 16. Additional Regression Methods

setting your convergence criterion too low, which can occur if your nonlinear specification is particularly complex.

If you wish to change the convergence criterion, enter the new value in the Options tab. Be aware that increasing this value increases the possibility that you will stop at a local minimum, and may hide misspecification or non-identification of your model.

See “Setting Estimation Options” on page 951, for related discussion.

Generalized Method of Moments (GMM)

The starting point of GMM estimation is a theoretical relation that the parameters should satisfy. The idea is to choose the parameter estimates so that the theoretical relation is satisfied as “closely” as possible. The theoretical relation is replaced by its sample counterpart and the estimates are chosen to minimize the weighted distance between the theoretical and actual values. GMM is a robust estimator in that, unlike maximum likelihood estimation, it does not require information of the exact distribution of the disturbances. In fact, many common estimators in econometrics can be considered as special cases of GMM.

The theoretical relation that the parameters should satisfy are usually orthogonality condi-

tions between some (possibly nonlinear) function of the parameters f( θ)

and a set of

instrumental variables zt :

 

E( f( θ) ′Z ) = 0 ,

(16.46)

where θ are the parameters to be estimated. The GMM estimator selects parameter estimates so that the sample correlations between the instruments and the function f are as close to zero as possible, as defined by the criterion function:

J( θ) = ( m( θ) ) ′Am( θ) ,

(16.47)

where m( θ) = f( θ) ′ Z and A is a weighting matrix. Any symmetric positive definite matrix A will yield a consistent estimate of q . However, it can be shown that a necessary (but not sufficient) condition to obtain an (asymptotically) efficient estimate of q is to set A equal to the inverse of the covariance matrix of the sample moments m . It is worth noting here that in standard single equation GMM, EViews estimates a model with the objective function (16.47) divided by the number of observations. For equations with panel data (“GMM Estimation” beginning on page 905) the objective function is not similarly scaled.

Many standard estimators, including all of the system estimators provided in EViews, can be set up as special cases of GMM. For example, the ordinary least squares estimator can be viewed as a GMM estimator, based upon the conditions that each of the right-hand variables is uncorrelated with the residual.

Generalized Method of Moments (GMM)—489

Estimation by GMM in EViews

To estimate an equation by GMM, either create a new equation object by selecting Object/ New Object.../Equation, or press the Estimate button in the toolbar of an existing equation. From the Equation Specification dialog choose Estimation Method: GMM. The estimation specification dialog will change as depicted below.

To obtain GMM estimates in EViews, you need to write the moment condition as an orthogonality condition between an expression including the parameters and a set of instrumental variables. There are two ways you can write the orthogonality condition: with and without a dependent variable.

If you specify the equation either by listing variable names or by an expression with an equal

sign, EViews will interpret the moment condition as an orthogonality condition between the instruments and the residuals defined by the equation. If you specify the equation by an expression without an equal sign, EViews will orthogonalize that expression to the set of instruments.

You must also list the names of the instruments in the Instrument list field box of the Equation Specification dialog box. For the GMM estimator to be identified, there must be at least as many instrumental variables as there are parameters to estimate. EViews will always include the constant in the list of instruments.

For example, if you type,

 

 

 

Equation Specification: y

c

x

Instrument list:

c

z

w

the orthogonality conditions given by:

490—Chapter 16. Additional Regression Methods

 

Σ( yt c( 1 ) − c( 2) xt) = 0

 

 

Σ( yt c( 1) − c( 2 )xt) zt

= 0

(16.48)

 

Σ ( yt c( 1 ) − c( 2 ) xt)wt = 0

 

If you enter the specification,

 

 

Equation Specification: c(1)*log(y)+x^c(2)

 

 

Instrument list:

c z z(-1)

 

 

the orthogonality conditions are:

 

 

 

Σ( c( 1 )log yt + xtc(2))

= 0

 

 

c(2)

 

(16.49)

 

Σ( c( 1 )log yt + xt )zt

= 0

 

 

 

Σ( c( 1) log yt + xtc(2) )zt − 1

= 0

 

On the right part of the Equation Specification dialog are the options for selecting the weighting matrix A in the objective function. If you select Weighting Matrix: Cross section (White Cov), the GMM estimates will be robust to heteroskedasticity of unknown form.

If you select Weighting Matrix: Time series (HAC), the GMM estimates will be robust to heteroskedasticity and autocorrelation of unknown form. For the HAC option, you have to specify the kernel type and bandwidth.

The Kernel Options determine the functional form of the kernel used to weight the autocovariances in computing the weighting matrix.

The Bandwidth Selection option determines how the weights given by the kernel change with the lags of the autocovariances in the computation of the weighting matrix. If you select Fixed bandwidth, you may either enter a number for the bandwidth or type nw to use Newey and West’s fixed bandwidth selection criterion.

The Prewhitening option runs a preliminary VAR(1) prior to estimation to “soak up” the correlation in the moment conditions.

The technical notes in “Generalized Method of Moments (GMM)” on page 716 describe these options in greater detail.

Example

Tauchen (1986) considers the problem of estimating the taste parameters β , γ from the Euler equation:

( βRt + 1wtγ+ 1 − 1 ) ′zt = 0

(16.50)

Generalized Method of Moments (GMM)—491

where we use instruments zt = ( 1, wt, wt − 1, rt, rt − 1) ′ . To estimate the parameters β , γ by GMM, fill in the Equation Specification dialog as:

Equation Specification: c(1)*r(+1)*w(+1)^(-c(2))-1

Instrument list: c w w(-1) r r(-1)

The estimation result using the default HAC Weighting Matrix option looks as follows:

Dependent Variable: Implicit Equation

Method: Generalized Method of Moments

Date: 09/26/97 Time: 14:02

Sample(adjusted): 1891 1982

Included observations: 92 after adjusting endpoints

No prewhitening

Bandwidth: Fixed (3)

Kernel: Bartlett

Convergence achieved after: 7 weight matricies, 7 total coef iterations

C(1)*R(+1)*W(+1)^(-C(2))-1

Instrument list: C W W(-1) R R(-1)

 

Coefficient

Std. Error

t-Statistic

Prob.

 

 

 

 

 

C(1)

0.934096

0.018751

49.81600

0.0000

C(2)

1.366396

0.741802

1.841995

0.0688

 

 

 

 

S.E. of regression

0.154084

Sum squared resid

2.136760

Durbin-Watson stat

1.903837

J-statistic

 

0.054523

 

 

 

 

 

Note that when you specify an equation without a dependent variable, EViews does not report some of the regression statistics such as the R-squared. The J-statistic reported at the bottom of the table is the minimized value of the objective function, where we report (16.47) divided by the number of observations (see “Generalized Method of Moments (GMM)” beginning on page 488 for additional discussion). This J-statistic may be used to carry out hypothesis tests from GMM estimation; see Newey and West (1987a). A simple application of the J-statistic is to test the validity of overidentifying restrictions when you have more instruments than parameters to estimate. In this example, we have five instruments to estimate two parameters and so there are three overidentifying restrictions. Under the null hypothesis that the overidentifying restrictions are satisfied, the J-statistic times the number of regression observations is asymptotically χ2 with degrees of freedom equal to the number of overidentifying restrictions. You can compute the test statistic as a named scalar in EViews using the commands:

scalar overid=eq_gmm.@regobs*eq_gmm.@jstat

scalar overid_p=1-@cchisq(overid,3)

where EQ_GMM is the name of the equation containing the GMM estimates. The second command computes the p-value of the test statistic as a named scalar OVERID_P. To view the value of OVERID_P, double click on its name; the value will be displayed in the status line at the bottom of the EViews window.

492—Chapter 16. Additional Regression Methods

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