tion is that it requires us to generalize from the data we already have to some other situation, often in the future; if our conclusions were limited to the specific people in the study at a particular time, then the study would not be very useful. In general, researchers must assume that their particular sample is representative of a larger <em>population</em>, which requires that they obtain the sample in a way that provides an unbiased picture of the population. For example, if the PURE study had recruit