title: Response to Hartwig and Davies.
authors.affiliation: MRC Integrative Epidemiology Unit, University of Bristol, Bristol, UK, jack.bowden@bristol.ac.uk.|MRC Biostatistics Unit, Cambridge Institute of Public Health, Cambridge, UK and.
authors.forename: Jack
authors.initials: J
authors.lastname: Bowden
authors.affiliation: Department of Public Health and Primary Care, University of Cambridge, Cambridge, UK.
authors.forename: Stephen
authors.initials: S
authors.lastname: Burgess
authors.affiliation: MRC Integrative Epidemiology Unit, University of Bristol, Bristol, UK.
authors.forename: George
authors.initials: G
authors.lastname: Davey Smith