Choosing effect measures and computing estimates of effect

A key early step in analysing results of studies of effectiveness is identifying the data type for the outcome measurements. This chapter considers outcome data of five common types: dichotomous (or binary) data, continuous data, ordinal data, count or rate data and time-to-event data. The ways in which the effect of an intervention can be assessed depend on the nature of the data being collected. For each of the types of data, the chapter reviews definitions, properties and interpretation of standard measures of intervention effect, and provides tips on how effect estimates may be computed from data likely to be reported in sources such as journal articles. Formulae to estimate effects for the commonly used effect measures are provided in a supplementary document statistical algorithms in Review Manager, as well as other standard textbooks. Effect measures are either ratio measures or difference measures. Ratio measures are typically analysed on a logarithmic scale.