Statistics: An introduction to sample size calculations

One crucial aspect of study design is deciding how big your sample should be. If you increase your sample size you increase the precision of your estimates, which means that, for any given estimate / size of effect, the greater the sample size the more “statistically significant” the result will be. In other words, if an investigation is too small then it will not detect results that are in fact important. Conversely, if a very large sample is used, even tiny deviations from the null hypothesis will be statistically significant, even if these are not, in fact, practically important. In practice, this means that before carrying out any investigation you should have an idea of what kind of change from the null hypothesis would be regarded as practically important. The smaller the difference you regard as important to detect, the greater the sample size required.