Sample size calculations for marketing testing

http://www.lucidview.com/sample_size.htm

Sample size is an important issue in marketing testing because it has such a large impact on the validity of your results and is so often misunderstood. Without sufficient sample size (i.e., sufficient data), your test will be meaningless–little more than a guess. Greater sample size increases your confidence that any change in response or sales is a real difference and not just random chance.