BASIC STATISTICS FOR CLINICIAN .3. ASSESSING THE EFFECTS OF TREATMENT- MEASURES OF ASSOCIATION

In the third of a series of four articles the authors show the calcula tion of measures of association and discuss their usefulness in clinic al decision making. From the rates of death or other ''events'' in exp erimental and central groups in a clinical trial, we can calculate the relative risk (RR) of the event after the experimental treatment, exp ressed as a percentage of the risk without such treatment. The absolut e risk reduction (ARR) is the difference in the risk of an event betwe en the groups. The relative risk reduction is the percentage of the ba seline risk (the risk of an event in the control patients) removed as a result of therapy. The odds ratio (OR), which is the measure of choi ce in case-control studies, gives the ratio of the odds of an event in the experimental group to those in the control group. The OR and the RR provide limited infer mation in reporting the results of prospectiv e trials because they do not reflect changes in the baseline risk. The ARR and the number needed to treat, which tells the clinician how man y patients need to be treated to prevent one event, reflect both the b aseline risk and the relative risk reduction. If the timing of events is important - to determine whether treatment extends life, For exampl e - survival curves are used to show when events occur over time.