# Panel data, simple rearrangement?

Consider the Correlated Random Effects model $y_{it} = \alpha + x_{it}\beta + \bar x \gamma + w_i + \epsilon_{it}$ where $x_{it}$ is a scalar explanatory variable.

The correlated random effects GLS estimator $\hat \beta_{CRE}$ is the OLS estimator of $\beta$ in the quasi-demeaned regression

$\tilde y_{it} = \delta + \tilde x_{it} \beta + \bar x_i \rho + u_{it}$,

where $\tilde y_{it} = y_{it} - \theta \bar y_i , \tilde x_{it} = x_{it} - \theta \bar x_i$ and $\theta = 1 - (\sigma^2_\epsilon/ (\sigma^2_\epsilon + T\sigma^2_w))^{1/2}$

Question: I need to show that the residuals from the regression of $x_{it} - \bar x_i$ on a constant and $\bar x_i$ is just $x_{it} - \bar x_i$ itself.

Attempt: Regress $x_{it} - \bar x_i = \alpha + \bar x_{i} + \tilde r_{it}$ rearrange to get the residuals, $\tilde r_{it} = (x_{it} - \bar x_i) - (\alpha + \bar x_{i})$ I'm not sure how to proceed.

By stnadard OLS regression results, in the simple regression

$$y_t = \alpha + \beta z_t + v_t, \;\;\;t=1,...,T$$

we have that

$$\hat \beta = \frac {\sum_{i=1}^T (z_t - \bar z)(y_t-\bar y)}{\sum_{i=1}^T (z_t - \bar z)^2}$$

and

$$\hat \alpha = \bar y - \hat \beta \bar z$$

So the residuals are

$$\hat v_t = y_t -(\hat \alpha +\hat \beta z_t)$$

Then, substitute for your particular $y_t$ and $z_t$ in all the above.

• Just a quick question: Since my explanatory variable is $\bar x_i$ (averaged over time), what is the value of the average of $\bar x_i$ Do I average over time again? (in which case the average of $\bar x_i$ would be itself). Or do i average over individuals i? (so that the average of $\bar x_i$ is $\bar x$?) I need this for the numerator of $\hat \beta$ – Kai_M Nov 24 '15 at 11:21
• @user44394 Wait a minute, I must be missing something here. If $\bar x_i$ is the time average per cross-section, then you cannot really regress each cross section separately "on a constant and $\bar x_i$", because both are constant,and so perfectly colinear, and we cannot obtain different estimates for each one. – Alecos Papadopoulos Nov 24 '15 at 16:09
• I completely agree. I think there could be a typo in my assignment sheet. This question is one part of a longer question which asks me to use the Frisch-Waugh theorem to show that $\hat \beta_{CRE} = \hat \beta_{FE}$. I found this paper: econ.msu.edu/faculty/wooldridge/docs/cre1_r4.pdf , and I am basically trying to prove proposition 2.1 (bottom page 12) but for balanced panels. The first part of the proof obtains the residuals from the regression of $x_{it} - \theta \bar x_i$ on $(1-\theta) \bar x_i$. So I guess there could be a typo in my assignment sheet here? – Kai_M Nov 24 '15 at 16:39