In this case, the sampling frame is not a list per se , but is rather a procedure that you follow as the actual basis for sampling. The sample is the group of people who you select to be in your study. Notice that I didn't say that the sample was the group of people who are actually in your study. You may not be able to contact or recruit all of the people you actually sample, or some could drop out over the course of the study. The problem of nonresponse and its effects on a study will be addressed elsewhere .
People often confuse what is meant by random selection with the idea of random assignment.
You should make sure that you understand the distinction between random selection and random assignment . At this point, you should appreciate that sampling is a difficult multi-step process and that there are lots of places you can go wrong. In fact, as we move from each step to the next in identifying a sample, there is the possibility of introducing systematic error or bias . For instance, even if you are able to identify perfectly the population of interest, you may not have access to all of them. And even if you do, you may not have a complete and accurate enumeration or sampling frame from which to select.