Meta-analysis for orthodontists: Part I--How to choose effect measure and statistical model.

Systematic reviews ideally provide a comprehensive and unbiased summary of existing evidence from clinical studies, whilst meta-analysis combines the results of these studies to produce an overall estimate. Collectively, this makes them invaluable for clinical decision-making. Although the number of published systematic reviews and meta-analyses in orthodontics has increased, questions are often raised about their methodological soundness. In this primer, the first steps of meta-analysis are discussed, namely the choice of an effect measure to express the results of included studies, and the choice of a statistical model for the meta-analysis. Clinical orthodontic examples are given to explain the various options available, the thought process behind the choice between them and their interpretation.