In general, if Yi is a response variable and Xi is a covariate of interest for i = 1,….. , K subjects, a regression model can be utilized to describe their relationship. In the case of longitudinal data, j is the index for the number of observations within a given subject. The number of repeated measurements on an individual will be represented as ni with nij being the measurement at the jth interval for the ith subject. Marginal models are based on quasi-likelihood and are similar in form to the Generalized Linear Model (GLM) in that a link function (g), is used to specify a mathematical relationship, involving regression coefficients (β), between a marginal mean response (μij), and one or more independent variables (Xij).
