“How many subjects do I need?” Neither7 nor 30 nor any number is an all-purpose answer. A sample size of 30 is a“large sample” in some textbook discussions of “normal approximation”; yet30,000 observations still may be too few to assess a rare, but seriousteratogenic effect. The best first response to “how many?” may be not a number,but a sequence of further questions. A study’s size and structure should dependon the research context, including the researcher’s objectives and proposedanalyses.