If you were to sample everybody and everything, that would be called a quota sample. Most research, however, involves non-quota samples. For example, if you were interested in state prison systems, you might sample 15 or so state prison systems. There are formulas for determining sample size, but the main thing is to be practical. Before gathering your sample, it's important to find out as much as possible about your population .
Population refers to the larger group from which the sample is taken.
This information will be needed later after you get to the data analysis part of your research, but it's also important in helping you decide sample size. The greater the diversity and differences that exist in your population, the larger your sample size should be. Capturing the variability in your population allows for more variation in your sample, and since many statistical tests operate on the principles of variation, you'll be making sure the statistics used later can do their powerful stuff. After you've learned all the theoretically important things about your population, you then have to obtain a list or contact information on those who are accessible or can be contacted. This procedure for listing all the accessible members of your population is called the sampling frame .