Field researchers are often interested in studying extreme or deviant cases – that is, cases that don’t fit into regular patterns of attitudes and behaviors. By studying the deviant cases, researchers can often gain a better understanding of the more regular patterns of behavior. This is where purposive sampling often takes place. For instance, if a researcher is interested in learning more about students at the top of their class, he or she is going to sample those students who fall into the "top of the class" category. They will be purposively selected because they meet a certain characteristic.
Purposive sampling can be very useful for situations where you need to reach a targeted sample quickly and where sampling for proportionality is not the main concern.
If a researcher is studying the nature of school spirit as exhibited at a school pep rally, he or she might interview people who did not appear to be caught up in the emotions of the crowd or students who did not attend the rally at all. In this case, the researcher is using a purposive sample because those being interviewed fit a specific purpose or description.
Researchers (typically market researchers) who you might often see at a mall carrying a clipboard and stopping various people to interview are often conducting research using purposive sampling. They may be looking for and stopping only those people who meet certain characteristics. For instance, if they are interested in the opinions of Hispanic females between 20 and 30 years old, they would stop the people passing by who look like they fit this description. One of the first things the researcher will do in this situation is verify that the respondent does in fact meet the characteristics or criteria for being included in the sample. If they do, the researcher will ask them the rest of the survey questions. If they do not meet the criteria, the researcher will likely send them on their way.
Babbie, E. (2001). The Practice of Social Research: 9th Edition. Belmont, CA: Wadsworth Thomson.