Читать книгу Practical Statistics for Nursing and Health Care - Jim Fowler - Страница 20

What interval is required to select a systematic sample of size 20 from a population of 800?

The required fixed interval is:

Therefore, the first patient (‘sampling unit’) is selected at random (as described in Section 2.8) from among patients numbered 1–40. Suppose number 23 is selected. The sample then comprises patients 23, 63, 103, 143, …, 783.

A disadvantage of systematic sampling occurs when the patients are listed in the population in some sort of periodic order, and thus we might inadvertently systematically exclude a subgroup of the population. For example, given a population of 800 patients listed by ‘first attendance’ at the clinic, and that over a 20‐week period, 40 patients registered per week, 20 during the daytime and 20 during the evening surgeries. If these patients were listed in the following order: Week 1 daytime patients, Week 1 evening patients, Week 2 daytime patients, …, Week 10 evening patients, then selecting patients 23, 63, …, 783 would result in a sample of evening clinic patients, and exclude all the daytime patients. It is possible that this could generate a biased, or unrepresentative, sample.

An argument in favour of systematic sampling occurs when patients are listed in the population in chronological order, say, by date of first attendance at the GP practice. A systematic sample would yield units whose age distribution is more likely to perfectly represent the study population.