Something noone else has said here, and I don't know the degree to which it is true for this study, but good study design generally includes stratified sampling

Something noone else has said here, and I don't know the degree to which it is true for this study, but good study design generally includes stratified sampling. e.g. if there are subpopulations whose experience differs a lot, you want to have those populations overrepresented in your study to reduce variance of study. This is basic statistics.