An example would be if we wanted to know how students' learning styles at the start of the medical course affect their final course results. The next step is to define exactly whom we are interested in studying. It is vital to ensure that this definition corresponds to the purposes of the survey. This usually includes specific personal criteria, time and place. For example, to determine the extent to which medical students in the United Kingdom read the studentBMJ , our study population may include all students who have registered for a medical degree in a United Kingdom university on a specified date.
Some surveys require more specific criteria.
Otherwise, the results would not give a valid answer to the original question. It would be helpful if we have some baseline information about this study population. For example, if our study population consists of all doctors in the United Kingdom on a particular date, we know the overall age, sex, ethnic, and specialist status of all doctors from various official sources - for example, the General Medical Council's Medical Register or the Medical Directory . This information is useful for at least two purposes. Sampling If we decide to collect information on the whole study population, the study is called a census.