A client's website was operating under a certain policy for membership sign ups for over a year. At the start of October 2012 the client implemented a new policy for sign ups that was supposed to encourage more registrations. This policy was applied site wide so there is no control group to compare against (except for the before period).
I have access to the entire history of sign ups and I am wondering if there is a test that I could use to determine if there is a statistically significant difference in the number of registrations before and since the policy change? Could I do something as simple as average sign ups per day before and after the policy change and do a t-test?