The comparison of the blood pressure in farmers and printers given in Chapter 3 is an example of a case control study. It is retrospective because we argued from the blood pressure to the occupation and did not start out with subjects assigned to occupation. There are many confounding factors in case control studies. A particular problem is recall bias, in that the cases, with the disease, are more motivated to recall apparently trivial episodes in the past than controls, who are disease free. Cross sectional studies are common and include surveys, laboratory experiments and studies to examine the prevalence of a disease.
Studies validating instruments and questionnaires are also cross sectional studies.
The study of urinary concentration of lead in children described in Chapter 1 and the study of the relationship between height and pulmonary anatomical dead space in Chapter 11 were also cross sectional studies. Sample size One of the most common questions asked of a statistician about design is the number of patients to include. It is an important question, because if a study is too small it will not be able to answer the question posed, and would be a waste of time and money. It could also be deemed unethical because patients may be put at risk with no apparent benefit. However, studies should not be too large because resources would be wasted if fewer patients would have sufficed.