Purpose This paper aims to theorise and test a causal model of predominantly lean-driven quality improvement (QI) in the context of health-care clinical microsystems, examining the effects contextual factors in this setting have on improvement activity. Design/methodology/approach QI practitioners at a New Zealand District Health Board were surveyed on a range of contextual factors hypothesised to influence improvement outcomes. Survey responses were analysed via partial least squares path modelling to test the causal model that was designed to be consistent with the “model for understanding success in quality” (MUSIQ) model (Kaplan et al., 2012) adopted in health-care QI. Findings Defined variables for teamwork, respect for people, lean actions and negative motivating factors all demonstrated significant effects. These findings support the representation of the microsystem layer within the MUSIQ model. The final model predicted and explained perceived success well (adjusted R2 = 0.58). Research limitations/implications The sample was a non-probability sample and the sample size was small (n = 105), although power analysis indicated that we exceeded the minimum sample size (97 cases). Even though health-care processes have universality, this study was conducted in only one district in New Zealand. Practical implications The results support highly functional teamwork as the critical contextual factor in health-care QI outcomes and suggest lean-driven process improvement can be a valid mediating mechanism. The key recommendation for practitioners is to increase focus on human resource capability when initiating and supporting QI. Originality/value The originality is testing the robustness of the MUSIQ model specifically in a lean environment, which provides the context for QI. The paper provides a more detailed specification of contextual factors acting as exogenous variables that moderate the cause (lean actions) and the effect (perceived success).