Last week we discussed how to determine sample size, or how many people we should approach to participate in our survey research efforts. Let's continue the conversation now with some more factors to consider when creating our sample size:
Cost of your research effort: Usually everyone has to make a trade-off between statistical accuracy of our data and our research budget or costs. The larger your sample is, the more precise your results will be, however it also means higher cost. Factors such as low incidence and low response rates can also increase sample cost. Many times you can draw a nice balance between the statistical accuracy of your results and your budget while compromising very little.
Your margin of error: Also called sampling error, the larger your sample, the smaller the margin of error and the greater your estimate of the data's precision, or correctness.
Your population size: Most of the time the size of the total target population is unknown, and it is assumed to be large (i.e. N=100,000 or larger), but in studies where the sample is a large fraction of the population of interest, some adjustments to your sample size are needed. Since most studies work with sub-samples, reviewing survey work that has been done in the past on those same populations can save you a lot of time, effort and money in determining your sampling needs.
Keeping these factors in mind, in addition to other survey sampling issues we've discussed, will ensure you draw the proper sample size for your research effort and that your results will be reliable and valid!