The sheer size of this undertaking is so vast that it only takes place once every 10 years. Infrequently, the whole population of the UK is presented with a series of forms to fill in, asking for information about their characteristics. Most of us are familiar with the national census. At the same time, we do not ignore qualitative perspectives on sampling, and you will see these are mentioned throughout this chapter. We also believe that the notion of sampling is not peculiar to research, but is part of our everyday lives, and many of our everyday concepts of sampling are most easily understood from a quantitative perspective. Even when a qualitative study is reported in the literature, the sampling technique may still be one initially derived from quantitative approaches. We start from a quantitative perspective because this is the one most commonly encountered in medical and healthcare research studies.

In this chapter, basic notions of the rationale behind sampling are explored. The specifics of different sampling techniques used in qualitative and quantitative approaches are examined in detail in Chapters 7 and 13. However, perhaps the most basic distinction in sampling in research is the one between sampling in quantitative and qualitative research. In the same way, the different approaches and aims are generally associated with different ways of sampling. In Chapter 3, we noted that different methodological approaches often have different research aims associated with them. This chapter introduces the ideas of reliability and validity in the context of what it means to sample from a wider population.

Quantitative and qualitative researchers place different emphasis on different aspects of validity and reliability.

Reliability is that it does so in a systematised, repeatable way.

Validity is the extent to which a study examines the entity it says it does.

Sampling technique is guided by the aim of the research.

Representativeness is the key to sampling, but is defined differently by quantitative and qualitative researchers.

Some populations are themselves samples from larger populations.

A population is a total group from which a sample is drawn.

Sampling is an everyday activity, not peculiar to research.

Reliability of sample