title: Petersen et al. Respond to "Effect Modification by Time-varying Covariates"
authors.affiliation: Division of Biostatistics, School of Public Health, University of California, Berkeley, CA.
authors.forename: Maya L
authors.initials: ML
authors.lastname: Petersen
authors.forename: Mark J
authors.initials: MJ
authors.lastname: van der Laan