Single-Factor Covariance Model

Let us consider a single covariate and one qualitative factor in a fixed effects model. The basic model in regression is 

However, many statisticians favor writing the model equation in ANOVA form  

1 23456789 10 (c) Group on test product hospital C FIGURE 11.1 Baseline counts among subjects in three different hospitals. 

This is a crucial requirement with ANCOVA, which sometimes cannot be met. If the treatment slopes are not parallel— that is, they interact—do not use ANCOVA. To test for parallelism, perform a parallelism test or plot out the covariates to determine this. If the slopes are not parallel, perform separate 

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As with other approaches, the propensity score model is only as good as the covariates selected to provide the adjustment. A propensity score is a single metric that is intended to account for all of the explanatory variables that predict who will receive treatment. Propensity scores generally balance observed confounders but do not necessarily produce balance on factors not incorporated into the model. Such imbalances represent a particular problem... 

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