Reference to “qualitative” versus “quantitative” sampling methods abounds in everyday parlance as well as in some anecdotal articles on research sampling methods. But, strictly speaking, sampling methods are not quantitative or qualitative, they are either probabilistic or non-probabilistic. The fact that qualitative research usually uses non-probability sampling methods does not justify referring to those methods (e.g. purposive sampling and snowballing) as “qualitative sampling” methods. It is similarly misleading to refer to probability sampling methods as “quantitative sampling” methods. Indeed, most quantitative research studies also employ non-probability sampling techniques such as convenience and quota sampling.
A study does not become qualitative or quantitative solely because of the sampling method used. Rather, the main difference between qualitative and quantitative research is that the former is largely theory building while the latter is largely theory testing. This major difference is manifested in the form of data that is collected (i.e. words versus numbers) and in the data analysis methods used (i.e. statistical versus thematic). So, the research methodology is decided first based on the study’s analytical objectives and the appropriate sampling method (i.e. the way to go about drawing and studying a fraction of the target population) is decided thereafter, depending on the profile of the target population and other practical considerations. In my view, if the target population is large and easy to access (e.g. people who buy bread) and a probability sampling technique is easy to implement, this should be done irrespective of research methodology. By the same rule, non-probability sampling (e.g. convenience sampling) is more appropriate for finite and hard to find sub-populations and can be used regardless of research methodology.
There shouldn't be a blanket rule that precludes the use of probability sampling in qualitative research. For example, if I was conducting focus group discussions with people who use a given shopping mall, I could identify and recruit one or two people who shop at the mall and thereafter ask them to refer me to other people who shop at the same mall that they know (i.e. use the snowballing technique). But I could also post recruiters at the entrance to the shopping mall and instruct them to intercept every 3rd person coming through and invite them to take part in the focus group discussion (i.e. use systematic random sampling).
It would appear one of the reasons for the conceptual exclusion of probability sampling in qualitative research is that qualitative research findings are not meant for generalization to the population from which the sample was drawn. Admittedly, that is the case in most qualitative studies. But the question is - if the population of interest is finite enough (e.g. the shopping mall example given above) and a probability sampling technique can be employed to select the study participants, why can’t qualitative research findings be made generalizable too, within certain analytical parameters? Put differently, given some known advantages of probability sampling, what harm is there in adopting this sampling methodology in instances where it lends itself to qualitative studies too?
