Background

Including a succinct justification for your chosen sample size is important for the Research Ethics Committee to understand that a credible plan is in place, and importantly that participant involvement is required. A detailed academic defence of your approach is not necessary. You may wish to read this paper to find out more.

In research, a sample is a group of people, items or objects taken from a larger population.

There are two major types of sampling, that is, the process and method of selecting your sample: probabilistic and non-probabilistic.

Probabilistic sampling

Probabilistic sampling refers to any sampling method which involves researchers using some form of random selection of items or individuals, rather than deliberate choice. Examples of probabilistic sampling include random sampling, stratified sampling, and cluster sampling.

Non-probabilistic sampling

Non-probabilistic sampling methods involve researchers deliberately choosing or selecting items or individuals for the sample, based on specific criteria and/or the specific research objectives. Examples of non-probabilistic sampling include convenience sampling, purposive sampling, and snowball sampling.

Quantitative research

In quantitative research protocols, the rationale for your sample size should be explained. If appropriate, you should also explain whether you have conducted a formal sample size calculation. This information should be included in your ethics application.

Quantitative sample size calculations should demonstrate exactly what statistical test the calculation is powered for (usually relating to the primary hypothesis/research questions), power, alpha, and effect size.

Qualitative research

Sample size in qualitative research is very different from quantitative research – there are no power calculations. However, it is important to justify your sample size as part of planning your research and to be transparent when you are reporting the results of the study. Good practice is to justify your sample size with reference to your particular study rather than citing an external norm.

Further information

• Further information about sampling strategies can be found here
• Further information on sample size calculations can be found here - Sage Research Methods provide various resources which can be accessed via your University of Bath login.
• Information about sampling for survey research can be found here
• For further information and guidance on sample size in qualitative research springer and sage journals