Any clinical study with a goal of arriving at a definitive conclusion regarding the relative efficacy of a new treatment needs to know how many subjects are needed to arrive confidently at a definitive answer.
There are three possible answers to a clinical study evaluating the clinical effectiveness of a new treatment –
- the study has sufficient information to confidently conclude the new treatment is more effective than its comparator;
- the study has sufficient information to confidently conclude the new treatment is not more effective than its comparator; or
- there is insufficient information to conclude with confidence whether the new treatment is more effective than its comparator.
Conclusions 1) and 2) represent definitive decisions that can be made with an acceptably high level of confidence. It is possible before beginning a study to determine how many study subjects would be needed to arrive at a definitive conclusion. Conclusion 3) often results when sample size planning is not done and, at study end, the researcher finds out not enough subjects were studied to allow for a definitive conclusion.
A common misconception is that 30 subjects per treatment arm are adequate for many studies. The number 30 is important in the statistics world but not for sample size decisions. Many studies need hundred, even thousands, of study subjects to reach their goal, whereas some studies (such as crossover or tightly controlled lab studies) may only need 20 subjects per treatment arm. The needed sample size for each study is uniquely determined by:
- the primary research objective,
- primary study endpoint,
- expected relative efficacy of the investigational treatment relative to its comparator, and
- measures of confidence desired for the resulting conclusion.