This module introduces some basic statistics for dealing with dichotomous outcomes. We discuss ways of summarising results and determining treatment effects within single trials, and prepare for meta-analysis.

Learning objectives

• Understand the difference between risk and odds
• Be able to calculate risk ratios and odds ratios from a 2×2 table for a single trial
• Be able to calculate risk differences and numbers needed to treat (NNT) from a 2×2 table for a single trial
• Be able to report and interpret risk ratios, odds ratios, risk differences and NNTs and their confidence intervals
• Be able to choose a suitable summary statistic for the meta-analysis and reporting of dichotomous data

Relevant sections of the Reviewers' Handbook

• Section 8 - 'Effect measures for single studies'

Where does this go in a Cochrane review?

The information in this module will be relevant to many parts of your review

• In the data analysis part of the Methods section of a protocol or review, where you will describe what statistical techniques you are planning to use
• When (or if) you actually perform meta-analyses using the analysis part of RevMan or other software
• In the presentation of results in the Results section of the review
• In the interpretation of results, in the Discussion of your review