Investigating the causal effect of socioeconomic status on quality of care under a universal health insurance system - a marginal structural model approach.

Income was shown to be a causal factor in a patient's health and a determinant of the quality of care received even with equitable access to care under a universal health insurance system. Policies focusing on addressing income as an important upstream causal determinant of health to provide support to patients in lower socioeconomic status will be effective in improving health outcomes for this vulnerable social stratum.