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. 2009 Dec 11;103(2):99–108. doi: 10.1093/qjmed/hcp169
Xij: the regression coefficients describing the effect of the covariates and the intercept,
β: the covariates (in this case the confounders) describing the patients characteristics of patient j in hospital i, including the constant term,
θ:i: the effect of hospital i, that is the coefficient with respect to some overall mean.