Heterogeneity statistic for inverse variance weighted (IVW) model and other related methods used in two-sample MR settings.
If the Q statistic much larger than its degrees of freedom (i.e., the number of instrumental variables (IVs) minus 1), this provides evidence for heterogeneity and invalid IVs. The Individual contribution of each IV to Q can be used to detect individual invalid IVs. This test statistic also assumes genetic IVs are uncorrelated and an extended version of the Q statistic is necessary when genetic IVs are weak.
References
- Greco M FD, Minelli C, Sheehan NA, Thompson JR. Detecting pleiotropy in Mendelian randomisation studies with summary data and a continuous outcome. Statistics in Medicine 2015; 34: 2926-2940.
- Bowden J, Del Greco M F, Minelli C, et al. Improving the accuracy of two-sample summary-data Mendelian randomization: moving beyond the NOME assumption. International Journal of Epidemiology 2018; 48: 728-742.