The differences between sampling in quantitative and qualitative research

The selection of a sample in quantitative and qualitative research is guided by two opposing philosophies. In quantitative research you attempt to select a sample in such a way that it is unbiased and represents the population from where it is selected. In qualitative research, number considerations may influence the selection of a sample such as: the ease in accessing the potential respondents; your judgement that the person has extensive knowledge about an episode, an event or a situation of interest to you; how typical the case is of a category of individuals or simply that it is totally different from the others. You make every effort to select either a case that is similar to the rest of the group or the one which is totally different. Such considerations are not acceptable in quantitative research.

The purpose of sampling in quantitative research is to draw inferences about the group from which you have selected the sample, whereas in qualitative research it is designed either to gain in-depth knowledge about a situation/event/episode or to know as much as possible about different aspects of an individual on the assumption that the individual is typical of the group and hence will provide insight into the group.

Similarly, the determination of sample size in quantitative and qualitative research is based upon the two different philosophies. In quantitative research you are guided by a predeter­mined sample size that is based upon a number of other considerations in addition to the resources available. However, in qualitative research you do not have a predetermined sample size but during the data collection phase you wait to reach a point of data saturation. When you are not getting new information or it is negligible, it is assumed you have reached a data saturation point and you stop collecting additional information.

Considerable importance is placed on the sample size in quantitative research, depending upon the type of study and the possible use of the findings. Studies which are designed to formulate policies, to test associations or relationships, or to establish impact assessments place a considerable emphasis on large sample size. This is based upon the principle that a larger sample size will ensure the inclusion of people with diverse backgrounds, thus making the sample representative of the study population. The sample size in qualitative research does not play any significant role as the purpose is to study only one or a few cases in order to identify the spread of diversity and not its magnitude. In such situations the data saturation stage during data collection determines the sample size.

In quantitative research, randomisation is used to avoid bias in the selection of a sample and is selected in such a way that it represents the study population. In qualitative research no such attempt is made in selecting a sample.You purposely select ‘information-rich’ respondents who will provide you with the information you need. In quantitative research, this is considered a biased sample.

Most of the sampling strategies, including some non-probability ones, described in this chapter can be used when undertaking a quantitative study provided it meets the require­ments. However, when conducting a qualitative study only the non-probability sampling designs can be used.

Source: Kumar Ranjit (2012), Research methodology: a step-by-step guide for beginners, SAGE Publications Ltd; Third edition.