This claim, although intuitively appealing, is untested. Decision making at general practice level about the level of resources to devote to health promotion and cardiovascular disease prevention would be enhanced if there were a reliable, valid method to evaluate the outcome, at the level of a single practice, of such endeavours. A point estimate of this, together with upper and lower bounds, is made from a table of absolute risks. These three measures are each summed over the patients in the sample. This process is repeated at regular intervals and the sums plotted on a control chart or charts.
Control charts are the key tool of statistical process control.
He was concerned with controlling the variation in manufacturing processes. They can be used to detect a change in the mean of a process. Joiner,8 in a modern treatment of control charts, sets five successive increases or decreases as statistically significant evidence of change. A retrospective cohort study design was used for this project. This printout also recorded the date the patient had last been seen.