I have 4 independent samples. Two groups A and B measured on time1 and two groups A and B measured on time2. For every group I calculate a proportion. I would like to compare the difference between the proportions over time for group A with the difference between the proportions over time for group B. To give an example:
- group A/time1 = 7%, group A/time2 = 14%; difference over time for group A = 14-7 = +7%
- group B/time1 = 12%, group B/time2 = 9%; difference over time for group B = 9-12 = -3%
(the samplesize in the 4 groups varies from 85 to 149) What test should I use to test whether the +7% is significantly different from the -3%?