ABSTRACT
Many individual clinical trials are not large enough to answer the questions we want to answer as reliably as we would want to answer them. (The same applies to epidemiological studies, but in this chapter we shall concentrate on clinical trials.) For example, trials are often too small for adequate conclusions to be drawn about potentially small advantages of particular therapies. Advocacy of large trials is a natural response to this situation, but it is not always possible to launch very large trials before therapies become widely accepted or rejected prematurely.
