Purposive sampling is using knowledge of the study and the population to choose participants. It is not a random sampling that looks at the whole population. Purposive sampling is also called judgmental sampling and selective sampling.
Purposive sampling is essential when researchers are studying a specific characteristic, feature or function. For example, if the researchers are studying the effects of asthma medication, then they are going to sample from only the population of people with asthma. This ensures that the research returns relevant information and avoids wasting time taking samples that have nothing to do with the research topic.
This type of sampling results in a group of responses that is not representative of the entire population, but instead represents a group that has a specific characteristic in common. Market research is a common example of purposive sampling because these researchers look almost exclusively at people who use a particular product or use a specific store or company.
The main disadvantage of purposive sampling is that it is prone to researcher bias because the samples are taken from such a specific group. The subjectivity and bias within purposive sampling can lead to data that is not trustworthy or accurate and must be redone.