Reflections on the I-squared index for measuring inconsistency in meta-analysis.

Area of Science:

• Biostatistics
• Medical Research Methodology

Background:

• The I-squared index was introduced in 2002 to evaluate consistency in meta-analysis results.
• It addresses limitations of the chi-squared test and random-effects models for heterogeneity.

Purpose of the Study:

• To discuss the definition, computation, and interpretation of the I-squared index.
• To introduce a novel interpretation of I-squared for simulation studies.
• To review extensions and provide recommendations for practical use.

Main Methods:

• Conceptual discussion and review of the I-squared index.
• Introduction of a new interpretation based on weighted sum of squares.
• Analysis of proposed extensions and repurposing of the index.

Main Results:

• I-squared quantifies the proportion of total variability due to true heterogeneity.
• It is often misinterpreted as an absolute measure of heterogeneity.
• A new interpretation as a weighted sum of squares is proposed.

Conclusions:

• The I-squared index is a valuable tool for meta-analysis but requires correct interpretation.
• Recommendations are provided for its appropriate application in research.
• Understanding its properties is crucial for accurate assessment of study consistency.