Quality of life researchers often only interview a sample of people instead of a whole population (a census). This has several advantages in terms of lower costs, greater depth, and the measurement of more variables. When taking a sample of a population, however, researchers need to be sure that the sample is also genuinely representative of that group.
In many cases, quality of life researchers use convenience samples, snowball samples, or quota samples. However, these do not allow the making of any statistical inference about the population from which it is taken, as they are not based on probability theory. To come to conclusions that are representative for a population, probability sampling is required, where each member of a population has known probability of being...