Many researchers feel that there is nothing more important than selecting a sample that accurately reflects the characteristics of the population they are interested in studying. Yet sample selection can sometimes be a risky business, with all kinds of questions needing to be answered before you can make any moves toward the sample selection process. Here is a list of factors to keep in mind:
Similar to the previous point, the population must match the characteristics of those groups you want to study. It might go without saying (but I’ll say it here anyway), but selecting a sample from a poorly identified population is the first major error in sample selection. If you want to study preschoolers, you cannot study first graders just because the two groups are close in age. The preschool and the first-grade experience differ substantially.The type of research you do will depend on the type and size of sample you need. For example, if you are doing case study descriptive research, which involves long, intense interviews and has limited generalizability (which is not one of the purposes of the method), you will need very few participants in your sample. If you are doing a group differences study, you will need at least 30 participants for each group.A highly reliable test will yield more accurate results than a homemade essay exam. The less reliable and valid your instruments, the larger the sample size that will be required to get an accurate picture of what you want.Consider the amount of financial resources at your disposal. The more money (and resources in general) you have, the more participants you can test. Remember, the larger the sample (up to a point) the better, because larger samples come closer to approximating the population of which they are a part.The number of variables you are studying and the number of groups you are using will affect the sample selection process. If you are simply looking at the difference in verbal skills between males and females, you can get away with 25–30 participants in each group. If you add age (5- and 10-year-olds) and socioeconomic status (high and low), you are up to six different possible combinations (such as 5-year-old girls of high socioeconomic status) and up to 6 × 30, or 180, subjects for an adequate sample size.