# Testing small proportions

My data is made up of 4 groups, each with a large and identical sample size of 20,000. The number of successes in each group is fairly small:

Group             A          B           C          D
Sample         20,000     20,000      20,000     20,000
Successes       90         120         110         200
Pr(Success)     0.5%       0.6%        0.6%        1.0%


I've done a simple chi-square test and found a significant difference (hope that was right!). Is it now appropriate to then do t-tests or are the proportions too small and I should do a series of further chi-squared tests? (I'm generally unsure about if/ when t-test is appropriate for very small. large proportions even if np and nq >20).

Thanks!

• Just recalled that chi-square may not be most suitable for this data as samples are so large, is there a more suitable test to use? – Hatti Jan 9 '15 at 10:23