This is an excellent blog post that explains the process of determining sample size. There is also an ebook available for download that goes into further detail.
A good article that discusses not only the sample size you need, but also how many people a survey will need to be sent to in order to get those results.
Are you looking at the entire population, or just a sample?
Would looking at a cohort make more sense for your study?
A cohort is a single group followed from one fixed point in time to another fixed point in time. Example: Looking at a group of students when they are freshmen, then looking at them again when they are seniors.
