# test of significance for comparing change between two groups

What test for significance is best to use to test the difference between the change of a categorical variable in a control group and the change in a pilot group?

The variable had three categories (e.g. on-street parking, off-street parking, and garage parking). We want to see if the change of the percent of people who utilized each of these different types of parking in the our control area was statistically different than the change of the percent of people who utilized each in out pilot area.

As an example:

Parking on-street:

Pilot before = 209/737 = 28% Pilot after = 145/754 = 19% Net percentage point change = -9%

Control before = 297/713 = 42% Control after = 242/716 = 34% Net percentage point change = -8%

Was the change in the pilot area statistically different from the change in the control area?

I do not think I can simply compare the 'after' for each because the 'before' for the two groups was statistically different - they did not start out the same so I believe I can only compare the change within each group.

I used the two-tailed, two-proportion z-test for within group differences.