Randomised controlled trials (RCTs)--sample size: the magic number (SMJ Vol 44 Issue 4 April 2003).

Correspondence to: Y H Chan Tel: (65) 6317 2121 Fax: (65) 6317 2122 Email: chanyh@ cteru.gov.sg INTRODUCTION A common question posed to a biostatistician from a medical researcher is “How many subjects do I need to obtain a significant result for my study?”. That magic number! In the manufacturing industry, it is permitted to test thousands of components in order to derive a conclusive result but in medical research, the sample size has to be “just large enough” to provide a reliable answer to the research question. If the sample size is too small, it’s a waste of time doing the study as no conclusive results are likely to be obtained and if the sample size is too large, extra subjects may be given a therapy which perhaps could be proven to be non-efficacious with a smaller sample size. Another major reason, besides the scientific justification for doing a study, why a researcher wants an estimate of the sample size is to calculate the cost of the study which will determine the feasibility of conducting the study within budget. This magic number will also help the researcher to estimate the length of his/her study – for example, the calculated sample size may be 50 (a manageable number) but if the yearly accrual of subjects is 10 (assuming all subjects give consent to be in the study), it will take at least five years to complete the study! In that case a multicentre study is encouraged.