# Cohen's d calculation when within group variance is zero

We have a single-case experimental design and we calculated Cohen's d for effect size using the mean difference between pre and post, divided by pooled standard deviation. However, for one of the outcomes, the variance for pre test is zero and the variance for post test is zero, even though there is clearly a change in the mean scores of pre and post. What should we do in this situation? Do we need to use a different equation?

First, investigate why all observations in pregroup and one postgroup are equal. Maybe very low $n$? Are the data reals or integers? Since you use the pooled variance for the standard errors, calculating effect size should not be a problem.