What is purposive sampling?

Purposive sampling is used in research studies to select a specific group of individuals or units for analysis. This method is appropriate when the researcher has a clear idea of the characteristics or attributes they are interested in studying and wants to select a sample representative of those characteristics.

This type of sampling is often used in qualitative research, allowing the researcher to focus on specific areas of interest and gather in-depth data on those topics. In this article, we will explore the concept of purposive sampling in more detail and discuss the advantages and limitations of using this approach in research studies.

Purposive sampling is a technique used in qualitative research to select a specific group of individuals or units for analysis. Participants are chosen “on purpose,” not randomly. It is also known as judgmental sampling or selective sampling.

In purposive sampling, the researcher has a specific purpose or objective in mind when selecting the sample. Therefore, the sample is selected based on the characteristics or attributes that the researcher is interested in studying. 

For example, suppose a researcher is interested in studying the experiences of individuals living with chronic pain. In that case, they might use purposive sampling to select a sample of individuals who have been diagnosed with chronic pain.

Purposive sampling is often used in qualitative research, as it allows the researcher to focus on specific areas of interest and gather in-depth data on those topics. It is also commonly used in small-scale studies with limited sample size.

Purposive sampling should be used when you have a clear idea of the specific attributes you're interested in studying and want to select a sample that accurately represents those characteristics.

Purposive sampling can be particularly useful in the following situations:

• When the population of interest is small

• For interest in studying a specific subgroup within the population

• To study a rare or unusual phenomenon

It's important to note that purposive sampling is not suitable for all research studies and should be used cautiously. As the sample is not selected randomly, the results of the study may not be generalizable to the larger population, and the researcher must consider the potential for bias in the sample selection.

There are several important principles of purposive sampling that you should consider when using this approach in your research studies:

1. Clearly defined purpose - The purpose of the study should be clearly defined, and the sample should be selected based on the characteristics or attributes that you're interested in studying.

2. Representative sample - The sample should be representative of the characteristics or attributes being studied.

3. Bias - Biases can come into play when anything other than random sampling is used, so be aware of any potential biases and take steps to minimize them.

4. Expertise - Having expertise in the topic being studied is an important part of sample selection. Without a solid understanding of the characteristics being selected, the population might not be as representative as it should be.

The steps to conducting a study using purposive sampling will vary depending on the topic and preferences of the researchers involved. The five steps of purposive sampling as a general framework are:

1. Define the purpose of the study

2. Identify the sample of individuals or units

3. Obtain informed consent from individuals

4. Collect the data using appropriate research methods

5. Analyze the data

Researchers can use several different types of purposive sampling methods, depending on what they're interested in studying and the specific research question they are trying to answer. In the list below, we'll discuss the various types of purposive sampling methods and provide examples of when each method might be used in research.

Maximum variation sampling

Maximum variation sampling involves selecting a sample of individuals or units representing the maximum range of variation within the characteristics or attributes the researcher is interested in studying. This type of sampling is used to understand the widest possible diversity of experiences or viewpoints within the population.

Homogeneous sampling

Homogeneous sampling involves selecting what is often a more narrow sample of individuals or units that are similar or have the same characteristics or attributes. This type of sampling is used to study a specific subgroup within the population in depth.

Typical case sampling

Typical case sampling involves selecting a sample of individuals or units that are representative of the typical experiences or characteristics of the population. This type of sampling is used to understand the most common or average experiences or characteristics within the population.

Extreme/deviant case sampling

Extreme case sampling involves selecting a sample of individuals or units that are considered extreme or unusual in the characteristics or attributes the researcher is interested in studying. This type of sampling is used to understand unusual or exceptional experiences or characteristics within the population and are often viewed as outliers in a wider population.

Critical case sampling

Critical case sampling involves selecting a sample of individuals or units that are important or central to the research question or the population being studied. This type of sampling is used to understand key experiences or characteristics within the population.

Expert sampling

Expert sampling involves selecting a sample of individuals or units that have specialized knowledge or expertise in the topic or issue being studied. This type of sampling is used to gather insights and understanding from experts in the field, which can be used to develop follow-up studies.

Purposive sampling and convenience sampling are similar in that both involve the selection of a sample based on the researcher's judgment rather than using a random sampling method. However, there are some key differences between the two approaches.

In purposive sampling, the sample is selected based on the defined purpose of the study and is intended to be representative of the characteristics or attributes in which the researcher is interested.

Convenience sampling, on the other hand, involves selecting a sample of individuals or units that are readily available or easily accessible to the researcher. The sample is not selected based on any particular characteristics or attributes, but rather in terms of convenience for the researcher.

There are several advantages to using purposive sampling in research studies, including:

• Representative sample - allows the researcher to select a sample highly representative of the characteristics or attributes they are interested in studying, relatively quickly, This can be particularly useful when the population of interest is small or when the researcher is interested in studying a specific subgroup within the population.

• In-depth data - often used in qualitative research, which allows the researcher to gather in-depth data on specific topics or issues. This can provide valuable insights and understanding of the research question.

• Practicality - practical and efficient in comparison to other sampling methods, particularly in small-scale studies with limited sample sizes.

• Flexibility - flexibility in the selection of the sample, which can be useful when the researcher is studying a rare or unusual phenomenon.

• Cost - can be less expensive than other sampling methods, as it does not require a random selection process.

It's important to note that purposive sampling has limitations and should be used with caution. Some of the disadvantages of purposive sampling are listed below:

• Limited generalizability - As the sample is not selected randomly, the study’s results may not be generalizable to the larger population. Other risk factors are producing lop-sided research, where some subgroups are omitted or excluded.

• Bias - Purposive sampling is subjective and relies on the researcher's judgment, which can introduce bias into the study. The researcher may unconsciously select individuals or units that fit their expectations or preconceived notions, which can affect the study’s validity. Participants can also manipulate the insights they give.

• Sampling error - Sampling error, or the difference between the sample and the population, is more likely to occur in purposive sampling because the sample is not selected randomly. This can affect the reliability and accuracy of the study.

• Limited sample size - Purposive sampling is often used in small-scale studies with limited sample sizes. This can affect the statistical power of the study and make it more difficult to detect significant differences or relationships.

• Ethical considerations -  The researcher must ensure that the study is conducted ethically and that the rights of the participants are protected. This may require obtaining informed consent from the individuals in the sample and safeguarding their privacy.

One of the main challenges to the use of purposive sampling in research studies is the limited generalizability of the findings. Because the sample is not selected randomly, it may not be representative of the broader population, and study results may not be applicable to other groups or populations. This can limit the usefulness and impact of the study, making it more challenging to draw conclusions about the larger population.

Each of the disadvantages listed in the previous section contributes to this problem. Researchers who wish to use purposive sampling need to be aware of the method’s weaknesses and actively take steps to avoid or mitigate them.

Purposive sampling is used in research studies when the researcher has a clear idea of the characteristics or attributes they are interested in studying and wants to select a sample that is representative of those characteristics. It is often used in qualitative research to gather in-depth data on specific topics or issues.

An example of purposive sampling might be a researcher studying the experiences of individuals living with chronic pain, and therefore selecting a sample of individuals who have been diagnosed with chronic pain.

Purposive sampling is often used in qualitative research, as it allows the researcher to gather in-depth data on specific topics or issues. It may also be used in small-scale studies with a limited sample size.

The sample size for purposive sampling will depend on the research question and the characteristics or attributes the researcher is interested in studying. Generally, a sample size of 30 individuals is often considered sufficient for qualitative research, although larger sample sizes of 100 or more may be needed in some cases.