In a longitudinal study of dose-response, the presence of confounding or non-compliance compromises the estimation of the true effect of a treatment. Standard regression methods may not remove the bias introduced by patient-selected treatment level, that is, they may not permit the estimation of the causal effect of dose. In this talk, I will introduce the Generalized Propensity Score (GPS), a generalization of the classical, binary treatment propensity score, which can be used to construct a balancing score that provides an estimation procedure for the true effect of dose. I will also describe the multivariate extension of the GPS approach. The method is then used to investigate the dose-response relationship in an ophthalmological setting between occlusion and improvement in visual acuity.