1
$\begingroup$

Jeffrey M. Wooldridge Econometric Analysis of Cross Section and Panel Data

Chapter 4 The Single-Equation Linear Model and OLS Estimation

Section 4.2 Asymptotic Properties of OLS

Subsection 4.2.2 Asymptotic Inference Using OLS

Assumption OLS.3: $E\left(u^{2}x^{\prime}x\right)=\sigma^{2}E\left(x^{\prime}x\right)\text{ where }\sigma^{2}\equiv E\left(u^{2}\right)$

Eq. (4.10) states $\widehat{Avar\left(\hat{\beta}\right)}=\hat{\sigma}^{2}\left(X^{\prime}X\right)^{-1}$ where $\hat{\sigma}$ is a consistent estimator of $\sigma$

I think it should be $\widehat{Avar\left(\hat{\beta}\right)}=\hat{\sigma}^{2}\left(\frac{1}{N}X^{\prime}X\right)^{-1}$

which one is correct:

$\widehat{Avar\left(\hat{\beta}\right)}=\hat{\sigma}^{2}\left(\sum_{i=1}^{N}x_{i}^{\prime}x_{i}\right)^{-1}$

$\widehat{Avar\left(\hat{\beta}\right)}=\hat{\sigma}^{2}\left(\frac{1}{N}\sum_{i=1}^{N}x_{i}^{\prime}x_{i}\right)^{-1}$?

$\endgroup$

1 Answer 1

0
$\begingroup$

A copy of Wooldridge is not in proximity. But the author is correct in stating equation no. $(4.10). $

Because it seems to obfuscate the thinking in OP, it warrants a brief casting of light on the matter.

The variance of the least square coefficient vector $\bf b$ is

\begin{align} \operatorname{Var} [\mathbf b\mid \mathbf X] & =\left(\mathbf{ X^\top X}\right) ^{-1}\mathbf X^\top\mathbb E \left[\boldsymbol{\varepsilon\varepsilon}^\top\mid \mathbf X\right]\left(\left(\mathbf{ X^\top X}\right) ^{-1}\mathbf X^\top\right) ^\top\\ &= \sigma^2\left(\mathbf{ X^\top X}\right) ^{-1}.\tag 1\label 1 \end{align}

For estimation of $\eqref 1, $ one needs to estimate $\sigma^2.$ It is an easy routine to deduce based on sum of squared residuals

$$\mathbb E\left[\mathbf e^\top\mathbf e\mid \mathbf X\right] = (n - k) \sigma^2.\tag 2\label 2$$

From $\eqref 2 ,$ one gets an (unbiased) estimator of $\sigma^2$ and subsequently

$$\operatorname{Est.Var} [\mathbf b\mid \mathbf X] = s^2\left(\mathbf{ X^\top X}\right) ^{-1},\tag 3$$ where $$s^2 := \frac{\mathbf e^\top \mathbf e}{n-k}.$$

Now, to ensure the data in large samples is "well-behaved", one assumption taken is

\begin{align}\operatorname{plim} \frac{\mathbf X^\top \mathbf X}n &=: \mathbf Q \\&\equiv \textrm{a positive definite matrix};\tag{A1}\end{align} this is attainable if it is assumed $$\mathbb E\left[\mathbf x_i\mathbf x_i^\top\right] = \mathbf Q, $$ which means $\sum \mathbf x_i\mathbf x_i^\top/n \overset{\mathbb P}{\to}\mathbf Q$ and hence $\operatorname{plim}\left( \frac{\mathbf X^\top\mathbf X}n\right) ^{-1} =\mathbf Q^{-1}.$

Now, the asymptotic variance, like $\eqref 1 ,$ would be

$$\operatorname{Asy.Var}[\mathbf b] = (\sigma^2/n) \mathbf Q^{-1}.\tag 4\label 4$$ Again, to get an estimator of $\eqref 4, $ one needs to concentrate on the term in parenthesis - $s^2$ can be used with proper evaluation of its consistency:

\begin{align}s^2 &= \frac{\boldsymbol \varepsilon^\top\mathbf M\boldsymbol\varepsilon}{n-k}\\&= \frac{\boldsymbol \varepsilon^\top \boldsymbol\varepsilon - \boldsymbol\varepsilon^\top \mathbf X\left(\mathbf{ X^\top X}\right)^{-1}\mathbf X^\top\boldsymbol \varepsilon}{n-k}\\ &= \frac{n}{n-k}\left[\frac{\boldsymbol \varepsilon^\top\boldsymbol\varepsilon}n - \left(\frac{\boldsymbol\varepsilon^\top\mathbf X}n\right)\left(\frac{\mathbf{ X^\top X}}n\right)^{-1}\left(\frac{\mathbf X^\top\boldsymbol \varepsilon}n\right)\right]; \tag 5\label 5, \end{align} where $\mathbf M:= \mathbf I - \mathbf X\left(\mathbf{ X^\top X}\right)^{-1}\mathbf X^\top$ is the residual maker. Using $(\mathrm A1) $ and the fact that $\operatorname{plim} \left(\frac{\boldsymbol\varepsilon^\top\mathbf X}n\right) = \mathbf O$ (since $\mathbb E[\mathbf x\boldsymbol{\varepsilon}] =\mathbf O$), the second term in the bracket in $\eqref 5$ converges to $0.$ The factor outside the bracket converges to $1.$ The only term left for inspection is $\sum \varepsilon_i^2/n.$ Now $\varepsilon_i^2$ are independent with same finite expectation $\sigma^2.$ To ensure the term's convergence (almost surely to $\sigma^2$), Markov's law of large numbers could be employed provided $\mathbb E\left[\left(\varepsilon_i^2\right)^{1+\delta}\right] <\infty$ for some $\delta \in (0, 1) $ - it would suffice then to assume for all $\varepsilon_i,$ there exist finite moments greater than $2.$ In any case, $$\operatorname{plim} s^2 = \sigma^2. \tag 6$$ This implies

$$ \operatorname{plim} s^2 \left( \frac{\mathbf X^\mathsf T\mathbf X}n\right) ^{-1}= \sigma^2\mathbf Q^{-1}. \tag{ 6.I}$$ Therefore, the appropriate estimator of $(4)$ would be $$\operatorname{Est.Asy.Var}[\mathbf b] = s^2\left(\mathbf X^\top\mathbf X\right) ^{-1}.\tag 7$$

--

Reference:

Econometric Analysis, William H. Greene, Pearson Education, 2018.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.