By computing weights on the basis of stratified propensity scores, MMW-S adjustment equates the observed pretreatment composition of multiple treatment groups. Analyzing data from a weighted sample, researchers can estimate a causal effect by computing the difference between the estimated average potential outcomes associated with alternative treatments within the ANOVA framework.
Hong, G. (2012). Marginal mean weighting through stratification: A generalized method for evaluating multi-valued and multiple treatments with non-experimental data. Psychological Methods, 17(1), 44-60.