Sometimes instrumental variable methods are used to test whether a causal effect is null rather than to estimate the magnitude of a causal effect. However, when instrumental variable methods are applied to time-varying exposures, as in many Mendelian randomization studies, it is unclear what causal null hypothesis is tested. Here, we consider different versions of causal null hypotheses for time-varying exposures, show that the instrumental variable conditions alone are insufficient to test some of them, and describe additional assumptions that can be made to test a wider range of causal null hypotheses, including both sharp and average causal null hypotheses. Implications for interpretation and reporting of instrumental variable results are discussed.

Additional Metadata
Keywords Causal null hypothesis, Hypothesis testing, Instrumental variable, Mendelian randomization
Persistent URL dx.doi.org/10.1007/s10654-018-0396-6, hdl.handle.net/1765/113935
Journal European Journal of Epidemiology
Citation
Swanson, S.A, Labrecque, J. (Jeremy), & Hernán, M.A. (Miguel A.). (2018). Causal null hypotheses of sustained treatment strategies: What can be tested with an instrumental variable?. European Journal of Epidemiology, 33(8), 723–728. doi:10.1007/s10654-018-0396-6