For example, in a survey of back pain and its possible causes, the target population was all potential back pain sufferers. The study population was defined as all people aged 20-59 from eight communities, and a sample of subjects was then randomly selected for investigation from within this study population. With this design, inference from the study sample to the study population is free from systematic sampling error, but further extrapolation to the target population remains a matter of judgement. The definition of a study population begins with some characteristic which all its members have in common. Within this broad definition appropriate restrictions may be specified - for example in age range or sex.
Clinical observations determine decisions about individuals.
Epidemiological observations may also guide decisions about individuals, but they relate primarily to groups of people. This fundamental difference in the purpose of measurements implies different demands on the quality of data. An inquiry into the validity of death certificates as an indicator of the frequency of oesophageal cancer produced the results shown in Table 1.1 . Inaccuracy was alarming at the level of individual patients. Hence, in this instance, mortality statistics in the population seemed to be about right, despite the unreliability of individual death certificates.