Something came into force in one month. Did the months after it look different from the months before, by more than this metric moves anyway? With a comparison group that the change did not reach, whatever moved everyone drops out. Without one, the months before are all there is to judge against — and that is said, rather than glossed over.
Groups the change reached
At least one. One on its own can still be read before and after.
Groups it did not reach
The comparison. Leave empty to judge the months before against the months after, with nothing to rule out what else moved.
Pick a group the change reached.