Sample size is an element of research design that significantly affects the validity and clinical relevance of the findings identified in research studies. Factors that influence sample size include the effect size, or difference expected between groups or time points, the homogeneity of the study participants, the risk of error that investigators consider acceptable and the rate of participant attrition expected during the study. Appropriate planning in regard to each of these elements optimises the likelihood of finding an important result that is both clinically and statistically meaningful.