Difference in difference using spss I have 2 groups of patients (control and treated) and 1 variable measured at two time moments (before and after treatment).
Using paired T test I am able to determine the mean (with 95%CI) difference in each group (at baseline and after treatment). Using split-plot ANOVA I can say that the effect of the treatment is not different between the two groups (from the Test of Between Subjects Effects). But I am not able to determine the difference in difference (mean with 95%CI).
I forgot to say that I am using SPSS.
Thank you
 A: For difference in differences you can compute the mean of the outcome $Y$ for each group $g =$ {$C$ control, $T$ treated} in each period $t =$ {1 pre-treatment, 2 post-treatment},
$$(E[Y_{igt}|g=T, t=2] - E[Y_{igt}|g=T, t=1]) - (E[Y_{igt}|g=C, t=2] - E[Y_{igt}|g=C, t=1])$$
which you can do by hand with the corresponding if-statements in SPSS. If you need confidence intervals, a convenient way to achieve the same calculation as above is to run the OLS regression
$$Y_{it} = \alpha + \beta_1 \; \text{Treat}_i + \beta_2 \; \text{Post}_t + \beta_3 \; (\text{Treat $\cdot$ Post})_{it} + \epsilon_{it}$$
where $\text{Treat}_i$ equals one for individuals in the treatment group and is zero otherwise, and $\text{Post}_t$ equals one for the post-treatment period and is zero otherwise.
The parameter $\beta_3$ then estimates the difference in differences and the regression will provide you with standard errors, p-values, confidence intervals etc. All you need to do in SPSS is to create the treatment dummy, the second period dummy and their interaction, and then run the described regression.
