I've seen that Pearson's r is typically used for evaluation between non-experimental conditions. However, I've also seen Pearson's r used to indicate effect size - is this acceptable? Is it a useful descriptive statistic at least?
For example, there are two groups of students. In one group students are put into pairs to do a task, in the other they aren't. (This is a task whereby improvement is to be expected over time.) There is a moderate effect size between time and similarity between partners after being grouped. Is this enough: r = 0.25 for paired students over time, and 0.67, 0.4, 0.3 after grouping by pairs. (It seems almost beside the point to indicate that nonpaired individuals showed no significant similarity to everyone else over time.)