Consistent estimation in Mendelian randomization with some invalid instruments using a weighted median estimator

Jack Bowden, George Davey Smith, Philip C. Haycock, Stephen Burgess*

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

672 Citations (Scopus)
816 Downloads (Pure)


Developments in genome-wide association studies and the increasing availability of summary genetic association data have made application of Mendelian randomization relatively straightforward. However, obtaining reliable results from a Mendelian randomization investigation remains problematic, as the conventional inverse-variance weighted method only gives consistent estimates if all of the genetic variants in the analysis are valid instrumental variables. We present a novel weighted median estimator for combining data on multiple genetic variants into a single causal estimate. This estimator is consistent even when up to 50% of the information comes from invalid instrumental variables. In a simulation analysis, it is shown to have better finite-sample Type 1 error rates than the inverse-variance weighted method, and is complementary to the recently proposed MR-Egger (Mendelian randomization-Egger) regression method. In analyses of the causal effects of low-density lipoprotein cholesterol and high-density lipoprotein cholesterol on coronary artery disease risk, the inverse-variance weighted method suggests a causal effect of both lipid fractions, whereas the weighted median and MR-Egger regression methods suggest a null effect of high-density lipoprotein cholesterol that corresponds with the experimental evidence. Both median-based and MR-Egger regression methods should be considered as sensitivity analyses for Mendelian randomization investigations with multiple genetic variants.

Original languageEnglish
Pages (from-to)304-314
Number of pages11
JournalGenetic Epidemiology
Issue number4
Publication statusPublished - 7 Apr 2016


  • Egger regression
  • Instrumental variables
  • Mendelian randomization
  • Pleiotropy
  • Robust statistics

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