ABSTRACT
Longitudinal data arise when participants in a study are measured on the same variable (or variables) on several different occasions. Such data arise frequently in medical investigations, particularly from clinical trials. An example (taken from Davis 2002) is shown in Table 12.1. Here, 40 male subjects were randomly assigned to one of two treatment groups; each patient had been rated on the brief psychiatric rating scale (BPRS) measured before treatment (week 0) and at weekly intervals for 8 weeks. The BPRS assesses the level of 18 symptom constructs such as hostility, suspiciousness, hallucination, and grandiosity; each is rated from 1 (not present) to 7 (extremely severe). The scale is used to evaluate patients suspected of having schizophrenia. Longitudinal data can be analysed in a variety of ways ranging from the simple to the relatively complex. In this chapter, we concentrate on the former, leaving the latter until Chapters 13 and 14.
