What is the right sample size in quantitative research?

Image courtesy of: Glenn Carstens-Peters @glenncarstenspeters

The simple answer to this question is that it depends on how deeply you want to analyze the results. Here is an easy way to look at it, which is widely used in market research.

Sample sizes and depth of analysis

For example, you might do a simple survey with a sample size of 100 “completes” (i.e. 100 respondents who have completed the whole questionnaire).

But if you know that you will want to analyse the survey results for women separately to those for men, then you might decide you need 100 completes for each, giving a total sample size of 200 completes.

You may also decide you will want to analyse the results for younger and older respondents, split by males and females. So, you will want 100 completes in each group, giving a total required sample size of 400 completes.

This process, of deciding in advance how the results will need to be analysed, is widely used in quantitative research to decide on survey sample sizes.

The precision of results

Some researchers, who use the above approach, might decide that 100 completes per group-to-analyse will not give sufficiently precise results. They might choose to have 200 completes per group, for example, or more.

It is easiest to explain this with an example. Suppose you run a survey and ask respondents what is their favourite day of the week? And imagine 40% of them say “Saturday”. In reality, this result means a range around 40%, rather than exactly 40%. This is because the survey would give a slightly different result each time it is run with a new set of respondents, but the % saying “Saturday” will generally be somewhere in a range around 40%.

There is a statistical formula which tells us that with a sample size of 100 completes, the range around 40% would be 30%-50%. With a sample size of 200 completes, the range would be 33%-47%. In other words, the bigger the sample size, the narrower the range. Indeed, this is the very point of having larger survey sample sizes.

But survey respondents cost money. So researchers planning a survey need to think whether gain they will get in terms of narrower percentage ranges from larger sample sizes is worth the additional cost.

You can look at how the percentage range and sample size works using our survey sample size calculator, or just get in contact with me to explain it or to discuss your requirements.