Estimating with samples
In the fifth part of our series on medical statistics, Wai-Ching Leung takes us through sampling
The last article showed that an important use of medical statistics is to draw conclusions from observations.1 We cannot be absolutely certain about our conclusions, however, partly because it is impractical or impossible to collect information from all relevant subjects. For example, we may wish to compare how much money undergraduate students spend at one university compared with another. Although it is theoretically possible to survey all students in both universities, we do not have the time and resources to do so. A common approach is to survey a sample of, say, 500 students from each university and generalise our findings.
In generalising our results, we assume that the sample of subjects we collect information on (the study sample) is similar to, or representative of, the group of subjects we want to draw conclusions about (the study population). But this might not be true for two reasons. Firstly, our study