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I am trying to recreate the results in this table. The results have been obtained by difference in difference estimation. I can obtain values from all columns except for column 5 and 6.

Column 5 says that it is obtained by subtracting column 1 from column 2, and this is simple to do for the means, however I don't know of a way to add OLS regression standard errors of 2 groups together to give a combined standard error, and I don't know of any other way to estimate the combined standard errors.

Does anyone know if/how this can be done. Or does anyone have any advice on alternative methods for obtaining these values.

Thank you for any help.

Chris

tables i need to recreate

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Let $X_1$ be column 1, $X_2$ be column 2, $X_5$ be column 5.

Then

$$X_5 = X_2 - X_1$$ $$Var(X_5) = Var(X_1) + Var(X_2) -2Cov(X_1,X_2) =[SE(X_1)]^2 + [SE(X_2)]^2 -2Cov(X_1,X_2)$$

And $SE(X_5)=\sqrt{Var(X_5)}$.

So you need to get $Cov(X_1,X_2)$ in order to get $SE(X_5)$. Or you can prove that $X_1$ and $X_2$ are independent such that $Cov(X_1,X_2) =0$.

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