Sample size
Sample size refers to the number of people (the sample) in a given study. Sample size is important because the larger the sample size, the more reliable the results. For example, if you tested a new painkiller on a group of 100 people and found that three of them -- or 3% -- got severe headaches, you might suspect that the drug caused headaches as a side effect. However, if you did the same study with a sample size of 10,000 people and 300 of them developed headaches -- also 3% -- you would be much more certain that headaches were a real side effect of the drug and not a chance event that happened to occur in three individuals.