The PANEL Procedure

Parks Method (Autoregressive Model)

Standard Corrections

Parks (1967) considered the first-order autoregressive model in which the random errors ${u_{it} }$ , ${i=1, 2, {\ldots }, \mi {N} }$ , and ${t=1, 2, {\ldots }, \mi {T} }$ have the structure

$\displaystyle {E}( u^{2}_{it})$	$\displaystyle =$	$\displaystyle {\sigma }_{ii} \textrm{(heteroscedasticity)}$
$\displaystyle {E}(u_{it}u_{jt})$	$\displaystyle =$	$\displaystyle {\sigma }_{ij} \textrm{(contemporaneously ~ correlated)}$
$\displaystyle u_{it}$	$\displaystyle =$	$\displaystyle {\rho }_{i} u_{i,t-1}+ {\epsilon }_{it} \textrm{(autoregression)} \nonumber$

where

$\displaystyle {E}( {\epsilon }_{it})$	$\displaystyle =$	$\displaystyle 0$
$\displaystyle {E}( u_{i,t-1} {\epsilon }_{jt})$	$\displaystyle =$	$\displaystyle 0$
$\displaystyle {E}( {\epsilon }_{it} {\epsilon }_{jt})$	$\displaystyle =$	$\displaystyle {\phi }_{ij}$
$\displaystyle {E}( {\epsilon }_{it} {\epsilon }_{js})$	$\displaystyle =$	$\displaystyle 0 (s{\neq }t)$
$\displaystyle {E}( u_{i0})$	$\displaystyle =$	$\displaystyle 0$
$\displaystyle {E}( u_{i0} u_{j0})$	$\displaystyle =$	$\displaystyle {\sigma }_{ij}={\phi }_{ij}/(1- {\rho }_{i} {\rho }_{j}) \nonumber$

The model assumed is first-order autoregressive with contemporaneous correlation between cross sections. In this model, the covariance matrix for the vector of random errors u can be expressed as

$\displaystyle {E}( \Strong{uu} ^{})=\Strong{V} = \left[\begin{matrix} {\sigma }_{11}P_{11} & {\sigma }_{12}P_{12} & {\ldots } & {\sigma }_{1N}P_{1N} \\ {\sigma }_{21}P_{21} & {\sigma }_{22}P_{22} & {\ldots } & {\sigma }_{2N}P_{2N} \\ {\vdots } & {\vdots } & {\vdots } & {\vdots } \\ {\sigma }_{N1}P_{N1} & {\sigma }_{N2}P_{N2} & {\ldots } & {\sigma }_{NN}P_{NN} \\ \end{matrix} \nonumber \right]$

where

$\displaystyle P_{ij}= \left[\begin{matrix} 1 & {\rho }_{j} & {\rho }_{j}^{2} & {\ldots } & {\rho }^{T-1}_{j} \\ {\rho }_{i} & 1 & {\rho }_{j} & {\ldots } & {\rho }^{T-2}_{j} \\ {\rho }_{i}^{2} & {\rho }_{i} & 1 & {\ldots } & {\rho }^{T-3}_{j} \\ {\vdots } & {\vdots } & {\vdots } & {\vdots } & {\vdots } \\ {\rho }^{T-1}_{i} & {\rho }^{T-2}_{i} & {\rho }^{T-3}_{i} & {\ldots } & 1 \\ \end{matrix} \nonumber \right]$

The matrix V is estimated by a two-stage procedure, and $\bbeta$ is then estimated by generalized least squares. The first step in estimating V involves the use of ordinary least squares to estimate $\bbeta$ and obtain the fitted residuals, as follows:

$\hat{\mb {u}} =\mb {y}-\mb {X} \hat{\bbeta }_{OLS}$

A consistent estimator of the first-order autoregressive parameter is then obtained in the usual manner, as follows:

$\displaystyle \hat{\rho }_{i}= \left(\sum _{t=2}^{T} \hat{u}_{it} \hat{u}_{i,t-1}\right) ~ \bigg/~ \left(\sum _{t=2}^{T}{\hat{u}^{2}_{i,t-1}}\right) i=1, 2, {\ldots }, \emph{N} \nonumber$

Finally, the autoregressive characteristic of the data is removed (asymptotically) by the usual transformation of taking weighted differences. That is, for ${i=1,2,{\ldots },\mi {N} }$ ,

$y_{i1}\sqrt {1- \hat{\rho }^{2}_{i}}= \sum _{k=1}^{p}{X_{i1k}\mb {{\bbeta }}_{k}} \sqrt {1- \hat{\rho }^{2}_{i}} +u_{i1}\sqrt {1- \hat{\rho }^{2}_{i}}$

$y_{it}- \hat{\rho }_{i} y_{i,t-1} =\sum _{k=1}^{p}{( X_{itk}- \hat{\rho }_{i} \mb {X} _{i,t-1,k}) {\bbeta }_{k}} + u_{it}- \hat{\rho }_{i} u_{i,t-1} t=2,{\ldots },\mi {T}$

which is written

$y^{\ast }_{it}= \sum _{k=1}^{p}{X^{\ast }_{itk} {\bbeta }_{k}}+ u^{\ast }_{it} \; \; i=1, 2, {\ldots }, \mi {N} ; \; \; t=1, 2, {\ldots }, \mi {T}$

Notice that the transformed model has not lost any observations (Seely and Zyskind 1971).

The second step in estimating the covariance matrix V is applying ordinary least squares to the preceding transformed model, obtaining

$\hat{\mb {u}} ^{\ast }= \mb {y} ^{\ast }- \mb {X} ^{\ast } {\bbeta }^{\ast }_{OLS}$

from which the consistent estimator of ${\sigma }$ $_{ij}$ is calculated as follows:

$s_{ij}=\frac{\hat{\phi }_{ij}}{(1- \hat{\rho }_{i} \hat{\rho }_{j}) }$

where

$\hat{\phi }_{ij}=\frac{1}{(\mi {T} -p)} \sum _{t=1}^{T} \hat{u}^{\ast }_{it} \hat{u}^{\ast }_{jt}$

Estimated generalized least squares (EGLS) then proceeds in the usual manner,

$\hat{\bbeta }_{P}= ({\mb {X} ’}\hat{\mb {V}} ^{-1}\mb {X} )^{-1} {\mb {X} ’}\hat{\mb {V}} ^{-1}\mb {y}$

where $\hat{\mb {V}}$ is the derived consistent estimator of V. For computational purposes, ${\hat{\bbeta }_{P}}$ is obtained directly from the transformed model,

$\hat{\bbeta }_{P}= ({\mb {X} ^{\ast }}(\hat{\Phi }^{-1}{\otimes }I_{T}) \mb {X} ^{\ast })^{-1}{\mb {X} ^{\ast }} (\hat{\Phi }^{-1}{\otimes }I_{T}) \mb {y} ^{\ast }$

where ${\hat{\Phi }= [\hat{\phi }_{ij}]_{i,j=1,{\ldots },N} }$ .

The preceding procedure is equivalent to Zellner’s two-stage methodology applied to the transformed model (Zellner 1962).

Parks demonstrates that this estimator is consistent and asymptotically, normally distributed with

$\mr {Var}(\hat{\bbeta }_{P})= ({\mb {X} ’}\mb {V} ^{-1}\mb {X} )^{-1}$

Standard Corrections

For the PARKS option, the first-order autocorrelation coefficient must be estimated for each cross section. Let ${\rho }$ be the ${\mi {N} \times 1}$ vector of true parameters and ${R=(r_{1},{\ldots },r_{N}{)’} }$ be the corresponding vector of estimates. Then, to ensure that only range-preserving estimates are used in PROC PANEL, the following modification for R is made:

$r_{i} = \begin{cases} r_{i} & \mr {if} \hspace{.1 in}{|r_{i}|}<1 \\ \mr {max}(.95, \mr {rmax}) & \mr {if}\hspace{.1 in} r_{i}{\ge }1 \\ \mr {min}(-.95, \mr {rmin}) & \mr {if}\hspace{.1 in} r_{i}{\le }-1 \end{cases}$

where

$\mr {rmax} = \begin{cases} 0 & \mr {if}\hspace{.1 in} r_{i} < 0 \hspace{.1 in}\mr {or}\hspace{.1 in} r_{i}{\ge }1\hspace{.1 in} \forall i \\ \mathop {\mr {max}}\limits _{j} [ r_{j} : 0 {\le } r_{j} < 1 ] & \mr {otherwise} \end{cases}$

and

$\mr {rmin }= \begin{cases} 0 & \mr {if} \hspace{.1 in}r_{i} > 0 \hspace{.1 in}\mr {or}\hspace{.1 in} r_{i}{\le }-1\hspace{.1 in} \forall i \\ \mathop {\mr {max}}\limits _{j} [ r_{j} : -1 < r_{j} {\le } 0 ] & \mr {otherwise} \end{cases}$

Whenever this correction is made, a warning message is printed.