Sampling considerations in qualitative research

Previously I talked about the importance of developing a recruitment strategy when designing a research project. In this post, we provide a brief overview of sampling for qualitative research, but it is a huge and complicated issue.

Types of qualitative sampling

Marshall (1996) briefly describes three different approaches to qualitative sampling:

• Judgement/purposeful sampling: attempting to sample the best people to answer your research question.
• Theoretical sampling: your sampling method evolves as you gather more data and develop theories about it. This is commonly used as part of a grounded theory approach.
• Convenience sampling: sampling people who are the easiest to reach.

How to select a sampling approach

Before you choose any approach, you need to decide what you are trying to achieve with your sampling. Do you have a specific group of people that you need to have in your study, or should it be representative of the general population? Are you trying to discover something about a niche, or something that is generalizable to everyone? A lot of qualitative research is about a specific group of people, and Marshall notes:

“This is a more intellectual strategy than the simple demographic stratification of epidemiological studies, though age, gender and social class might be important variables. If the subjects are known to the research, they may be stratified according to known public attitudes or beliefs.”

Broadly speaking, convenience, judgement and theoretical sampling can be seen as purposeful – deliberately selecting people of interest in some way. However, randomly selecting people from a large population is still a desirable approach in some qualitative research. Qualitative studies tend to have a small sample size due to the in-depth nature of engagement with each participant, which can have an impact if you want a representative sample. If you randomly select 15 people, you might by chance end up with more women than men, or a younger than desired sample. That is why qualitative studies may use a little bit of purposeful sampling, finding people to make sure the final profile matches the desired sampling frame.

How large should my sample be for a qualitative study?

Sample size will often depend on conceptual approach: if you are testing a prior hypothesis, you may be able to get away with a smaller sample size, while a grounded theory approach to develop new insights might need a larger group of respondents to test that the findings are applicable to more people. Here, you are likely to take a ‘theoretical sampling’ approach (Glaser and Strauss 1967) where you specifically choose people who have experiences that would contribute to a theoretical construct. This is often iterative, in that after reviewing the initially collected data (for theoretical insights) the researcher goes out again to find other participants that the model suggests might be of interest.

What are the problems with convenience sampling?

The convenience sampling approach which Marshal mentions as being the ‘least rigorous technique’ is where researchers target the most ‘easily accessible’ respondents. This could even be friends, family or faculty. This approach can rarely be methodologically justified, and is unlikely to provide a representative sample. However, it is endemic in many fields, especially psychology, where researchers tend to turn to easily accessible psychology students for experiments: skewing the results towards white, rich, well-educated Western students.

What is snowball sampling?

Now we turn to snowball sampling (Goodman 1961). This is different from purposeful sampling in that new respondents are suggested by others. In general, this is most suited to work with ‘marginalised or hard-to-reach’ populations, where responders are not often forthcoming (Sadler et al 2010). For example, people may not be open about their drug use, political views or living with stigmatising conditions, yet often form closely connected networks. Thus, by gaining trust with one person in the group, others can be recommended to the researcher. However, it is important to note the limitations with this approach. Here, there is the risk of systemic bias: if the first person you recruit is not representative in some way, their referrals may not be either. So you may be looking at people living with HIV/AIDS, and recruit through a support group that is formed entirely of men: they are unlikely to suggest women for the study.

For these reasons there are limits to the generalisability and appropriateness of snowball sampling for most subjects of inquiry, and it should not be taken as an easy fix. Yet while many practitioners explain the limitations with snowball sampling, it can be very well suited for certain kinds of social and action research. This article by Noy (2008) outlines some of the potential benefits of
snowball sampling, particularly that it can give a sense of power relations and ”organic social networks”.

Sampling and data saturation in qualitative studies

Finally, there is the issue of sample size and ‘saturation’: when there is enough data collected to confidently answer the research questions. For a lot of qualitative research this means the data has been collected and coded, especially if using some variant of grounded theory. However, saturation is often a source of anxiety for researchers: see for example the amusingly titled article “Are We There Yet?” by Fusch and Ness (2015). Unlike quantitative studies where a sample size can be determined by the desired effect size and confidence interval in a chosen statistical test, it is more difficult to put an exact number on the right number of participant responses. This is especially because responses are themselves qualitative, not just numbers in a list: so one response may be more data rich than another.

While a general rule of thumb would indicate there is no harm in collecting more data than is strictly necessary, there is always a practical limitation, especially in resource and time constrained post-graduate studies. It can also be more difficult to recruit than anticipated, and many projects working with very specific or hard-to-reach groups can struggle to find a large enough sample size. This is not always a disaster, but may require a re-examination of the research questions, to see what insights and conclusions are still obtainable.

Generally, researchers should have a target sample size and definition of what data saturation will look like for their project before they begin sampling and recruitment. Don’t forget that qualitative case studies may only include one respondent or data point, and in some situations that can be appropriate. However, getting the sampling approach and sample size right is something that comes with experience, advice and practice.

As I always seem to be saying in this blog, it’s also worth considering the intended audience for your research outputs. If you want to publish in a certain journal or academic discipline, it may not be responsive to research based on qualitative methods with small or ‘non-representative’ samples. Silverman (2013 p424) mentions this explicitly with examples of students who had publications rejected for these reasons.

So as ever, plan ahead for what you want to achieve for your research project, the questions you want to answer, and work backwards to choose the appropriate methodology, methods and sample for your work.

Recommended reading on sampling

There’s a great chapter ‘Designing and Selecting Samples’ in the book Qualitative Research Practice (Ritchie et al 2013) which goes over many of these methods in detail.

Your research questions and methodological approach (such as grounded theory) will guide you to the right sampling methods for your study – there is never a one-size-fits-all approach in qualitative research! For more detail on this, especially on the importance of culturally embedded sampling, there is a well cited article by Luborsky and Rubinstein (1995). But it’s also worth talking to colleagues, supervisors and peers to get advice and feedback on your proposals.

Using qualitative analysis software after sampling

Once you have your data, Quirkos can be a great way to analyse it, whether your sample size has one or dozens of respondents! There is a free trial and example data sets to see for yourself if it suits your way of working.