when you conduct a survey, how do you know how many people is enough?
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I think all the answers here are missing two really important questions you need to consider.
First: how are you doing your sampling? People seem to be assuming you’re using a random sample. But that’s not the only way to do a sample. It’s usually regarded as a gold standard, but it’s not always the best or a feasible option. (As other people have said, it’s rarely possible to do a *truly* random sample. Very often bias in your sample is a bigger problem than sample size.)
Sample size can easily be a matter of looking at the results and feeling whether you’re likely to learn more by increasing it.
Second: how reliable and accurate do you need your results to be?
Other people have talked about how you can calculate the margin of error for a survey for a certain confidence level. But what’s the right margin of error for you? What’s the right confidence level? Do you need to be 90% confident or 99% confident?
That depends on how your research is going to be used. There are rules of thumb for this in science, though they’re not uncontroversial. I do research in a business context, where I’ll be asking questions like: how important is the decision you’re going to make? If you get it wrong, what are the consequences? Can you easily change things if you get them wrong? Do you have other evidence supporting your results?
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