ABSTRACT

This chapter introduces the cohort study and looks at risks and risk ratios, showing how confidence intervals are calculated. It discusses what a person-year is, and how it is connected to rates and rate ratios. The chapter describes the concept of bias, discusses the merits of a controlled, randomized, double-blind clinical trial and finally presents some comparisons between case control and cohort studies. Cohort studies usually require carefully planned, often lengthy investigations, whilst a case control study can quite often be performed quickly from a number of cases already collected. One important conceptual difference between the two types of study has to do with time: in a cohort study we start with a number of subjects who are free from disease, and follow them over time to see who becomes a case and who does not. In a modern randomized controlled trial, one would be very much aware that six arms with two subjects give a miserable statistical power.