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Originally Posted by RedHot25
My main question then for each of the 2 associations is (at a basic level) how much do I sample from them? Do I sample the same amount, or a percentage level? I.e. should my sample #s for the 5400 one be higher than the 1600 one? If I took a 50% sample of each one... 800 and (approx) 2500...ouch, that's a lot!
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I guess the answer depends on how important it is for you to have a statistically significant sample for each group. If you want to be accurate within +/- 5% at a .95 confidence level for each group, you'll need 310 responses from the group of 1600, and 360 from the group of 5400. If you're expecting a 25% reponse rate, you'll need to survey 1240 of the 1600, and 1440 of the 5400... which obviously is a lot. If you're happy with a +/- 10% confidence interval (obviously weaker), you could get away with ~100 answers from each group, or ~400 surveys to each.
If you're looking for the best data possible, given your limitations, I'd just pick a % from each group. Whether you choose the same % from each group is up to you - you could survey 10% of the larger group and 20% of the smaller group. However, you will need to be able to justify this when you present your findings (defend your thesis, etc.), and that could be a problem if you don't have a logical reason for doing so... and you can expect stats geeks to pick up on this (attacking the methodology is the easiest way to disprove findings).
One final thought -- have you considered an electronic (e-mail or web-based) survey? If you have valid e-mail addresses available, this is the most efficient way to collect your data as you can survey the entire population and expect a similar response rate to a written survey, without having the sample size questioned. There are companies that specialize in management of such surveys... they aren't cheap, but it might be comparable to the cost of printing & mailing 1000+ surveys... just a thought.