title: Identifying Causal Estimands for Time-Varying Treatments Measured with Time-Varying (Age or Grade-Based) Instruments.
abstract: This commentary discusses causal estimands of same-age and same-grade comparisons for assessing grade-retention effects on student ability and performance. Using potential outcomes notation, we show that same-age and same-grade comparisons refer to different retention-promotion contrasts and therefore assess different causal questions. We also comment on deleting versus censoring records of students who dropped out of the study or do not belong to the treatment regimes under investigation. Whereas deleting entire student records potentially induces collider bias, censoring circumvents bias if censoring is ignorable given the observed pretreatment covariates.
authors.affiliation: a University of Wisconsin - Madison.
authors.forename: Peter M
authors.initials: PM
authors.lastname: Steiner
authors.affiliation: b University of California - Riverside.
authors.forename: Soojin
authors.initials: S
authors.lastname: Park
authors.affiliation: a University of Wisconsin - Madison.
authors.forename: Yongnam
authors.initials: Y
authors.lastname: Kim
mesh_terms: D015982:Bias; D006801:Humans; D015233:Models, Statistical; D013334:Students
keywords: Causal inference; collider bias; same-age comparison; same-grade comparison; time-varying instruments; time-varying treatments