Let's say my survey population size is 1000 and I send a survey to all 1000. And only 200 responded. Is the 200 considered my sample size? If yes, can I calculate a margin of error on the questions in the responses?
Good question. I think it depends on what you are planning to do with that data. For a regression model or hypothesis tests, you'd probably consider 200 as the sample size as you are not including these non-responses in the model. On the other hand, if you are using something like a decision tree (or any machine learning extension like random forests), you might include missing responses as an explicit outcome (probably not for 800 out of 1000, but in principle you might). I believe in your use-case it would be 200.