Adverse effect

Unfavourable outcome that occurs during or after the use of a drug or other intervention for which the causal relation between the intervention and the event is at least a reasonable possibility (Loke 2011).

Available case analysis

Analysis that includes data on only those whose results are known, using as a denominator the total number of people who had data recorded for the particular outcome in question (Higgins 2011a).

Funnel plot

Simple scatter plot of the intervention effect estimates from individual studies against some measure of each study’s size or precision (Sterne 2011).

Intention‐to‐treat analysis

Analysis that fulfils the next principles:
1. keeps participants in the intervention groups to which they were randomised, regardless of the intervention they actually received;
2. there is a measurement of outcome data on all participants; and
3. includes all randomised participants in the analysis (Higgins 2011a).

Unit of analysis errors

An error made in statistical analysis when the analysis does not take account of the unit of allocation. In some studies, the unit of allocation is not a person, but is instead a group of people, or parts of a person, such as eyes or teeth. Sometimes the data from these studies are analysed as if people had been allocated individually. Using individuals as the unit of analysis when groups of people are allocated can result in overly narrow confidence intervals. In meta‐analysis, it can result in studies receiving more weight than is appropriate (Green 2005).

Small‐study effects

A tendency for estimates of the intervention effect to be more beneficial in smaller studies (Sterne 2011).