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Non-Probability Sampling

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What is non-probability sampling?

Research studies, often, are trying to find correlations, differences or characteristics of large groups of people. Often, it’s not viable to collect data from everyone in this group. In such cases, researchers must take it upon themselves to select an appropriate sample group to represent their target population. 

A sample group is a subset of a population. Studies will have a distinct target population from which the sample group must be selected. For example, if your research study is about the prevalence of depression in adolescents in France, your sample group must be selected from the population of adolescents in France.

It is important that researchers choose a sampling method appropriate to their study in order to choose their sample group. Choosing the right sample group is vital as the wrong sample group will not provide an accurate representation of their target population.

Sampling methods are divided into two broad categories, and they are as follows:

Probability Sampling: In probability sampling, a sample group is selected through a random and unbiased process.

Non-Probability Sampling: The method of non- probability sampling, on the other hand, involves processes of selecting a sample group through non-random processes, usually according to the researcher’s judgement. 

In this article, we will specifically be delving into non-probability sampling and its characteristics.

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Definition

Unlike probability sampling which uses methods of “random selection” to select a sample group, non-probability sampling is where the researcher uses their judgement to select this group. 
Hence, all members of a population (or target population) do not have an equal and known chance of being selected as a respondent.

Oftentimes non-probability sampling is used by researchers when probability sampling methods are not feasible. Probability sampling is better suited for quantitative research as it is more reflective of the larger population, however not all researchers may have the means to carry it out, and in such cases non-probability sampling is required.

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Types of Non-Probability Sampling

There are five main types of non-probability sampling:

  • Convenience Sampling
  • Quota Sampling
  • Purposive Sampling
  • Consecutive Sampling
  • Snowball Sampling

Convenience (or accidental) Sampling

As the name suggests, convenience sampling involves the collection of data where it is most readily available to the researcher. Convenience sampling is also known as accidental sampling because respondents are chosen whenever and wherever they are met. Convenience sampling is used in cases where certain target populations aren’t easily accessible, or when researchers have a limited time-frame.

For example, if a study about the consumption patterns of coffee is conducted, a researcher may choose to visit different coffee shops in order to collect information from customers at these shops. 

While employing the method of convenience sampling, it is important that the researcher avoids turning his sample group into a biased one as this will lead to inaccurate findings in the study.

Quota Sampling

Quota sampling is used in cases where the study aims to represent variables such as age, gender, income, occupation, or any other certain group. As this method of sampling involves the grouping of people with certain similar characteristics, it can be considered a form or proportionate stratified sampling (a component of probability sampling). However in this case, the predetermined proportion of people will be sampled from different groups based on convenience.

For example, if you want to conduct a study with 55% male participants, you may choose to include male participants readily available to you until you have reached the required number/size of your quota. 

Purposive (or judgemental) Sampling

As mentioned earlier, most researchers are interested in particular target populations. Purposive sampling, also known as judgemental sampling, is a method in which researchers choose respondents based on who is the best position to provide the required information. 

For example, if you were to conduct a study on the spending habits of millionaires, you would only be able to acquire first hand information from those who have built their networth into the millions, and therefore those people must be chosen as respondents.

 Snowball Sampling

Snowball sampling is a method of non-probability sampling which is used when the target population of the study is inaccessible or hard to find. In this case, the researcher contacts someone who meets the criteria to be included in their sample group, and then asks them to recommend other potential respondents who they may know who meet the criteria.

For example, if I’m doing a study on Native Americans, an ethnic group that is only 1.6% of the American population, it may be advantageous for me to use the snowball sampling method. After finding one or a few Native Americans, they may be able to give me a list of contacts of more Native Americans that they may know (relatives or friends, for instance).

Consecutive Sampling

Consecutive sampling, also known as total enumerative sampling, is a method of sampling where a researcher works with a sample group over a period of time, then picks a new sample group to work with again, and so on until they’ve acquired the amount of data they require. This is similar to convenience sampling, with the difference that there are multiple sample groups over the course of the study.

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When is Non-Probability Sampling used

  1. When researchers have a limited time frame to complete the study.
  2. When minority participation is required in the study.
  3. When certain target populations are inaccessible.
  4. When results of the study aren’t expected to generate results reflective of the whole target population.
  5. When a researcher may not have the funding or financial means to conduct probability sampling.

Advantages

  1. Compared to probability sampling, non-probability sampling is quick, convenient and less expensive.
  2. Sometimes using non-probability sampling is the only option as certain populations aren’t easily accessible or there is a time constraint for the completion of the study.
  3. It is the most effective form of sampling in studies where minority participation in the study is crucial.

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