Introduction

Before performing an observational or experimental study, it is ‘necessary’ to calculate the number of subjects that are needed to make sure that a predefined effect will be statistically significant. It is ‘necessary’ because sample-size calculations are a prerequisite for research grants, and ‘must’ be submitted to (medical) Ethical Committees. Furthermore, for experimental studies, sample-size calculations are part of the so-called CONSORT statement. This means that, without a sample-size calculation, a paper reporting on the results of an experimental study will not be published. The importance of sample-size calculations is a rather strange phenomenon. Firstly, sample-size calculations are based on many assumptions, which can easily be changed, and in which case the number of subjects needed, will be totally different. Secondly, sample-size calculations are (usually) based on statistical significance, which is strange, because in epidemiological and medical research the importance of significance levels is becoming more and more questionable. However, many people believe in the importance of sample-size calculations, and because ‘standard’ sample-size calculations are not appropriate in multilevel studies, specific sample-size calculations for multilevel studies will be discussed in this chapter.

There is a considerable amount of literature on sample-size calculations in multilevel studies (e.g. Snijders and Bosker, 1993; Lee and Durbin, 1994; Liu and Liang, 1997; Cohen, 1998; Plewis and Hurry, 1998; Hedeker et al., 1999; Moerbeek et al., 2000, 2003c; Jung et al., 2001).