Units of analysis

摘要

In clinical studies the focus of interest is almost always the patient. If we carry out a randomised trial to compare two treatments we are interested in comparing the outcomes of patients who received each of the treatments. In some conditions several measurements will be taken on the same patient, but the focus of interest remains the patient. Failure to recognise mis fact results in multiple counting of individual patients and can seriously distort the results. We explain this error below. Its frequency in medical research is indicated by the whole chapter devoted to it in Andersen's classic compilation.