Types of non-probability sampling

The first type is convenience sampling. The subjects are selected just because they are easiest to recruit for a study and the researcher does not consider selecting subjects that are representative of the entire population. In all forms of research, it would be ideal to test the entire population, but in most cases, the population is just too large that it is impossible to include every individual. This is the reason why most researchers rely on sampling techniques like convenience sampling, the most common of all sampling techniques. Many researchers prefer this sampling technique because it is fast, inexpensive, easy and the subjects are readily available.

Judgmental sampling or Purpose sampling - The researcher chooses the sample based on who they think would be appropriate for the study. This is used primarily when there are a limited number of people that have expertise in the area being researched. This type of sampling technique is also known as purposive sampling and authoritative sampling. Purpose sampling is used in cases where the specialty of an authority can select a more representative sample that can bring more accurate results than by using other probability sampling techniques. The process involves nothing but purposely handpicking individuals from the population based on the authorities or the researcher’s knowledge and judgment.

In this technique of sampling the first respondent refers a friend; the friend also refers a friend, (Castillo 2009). Snowball sampling is a non-probability sampling technique that is used by researchers to identify potential subjects in studies where subjects are hard to locate. Researchers use this sampling method if the sample for the study is very rare or is limited to a very small subgroup of the population. This type of sampling technique works like chain referral. After observing the initial subject, the researcher asks for assistance from the subject to help identify people with a similar trait of interest.