Effect means are least-squares estimates predicted by the model for each combination of levels in a categorical term, adjusted for the other model effects.
Least-squares means are predictions made at specific values of the predictors. For each categorical variable in the term, the value is the combination of levels. For all other variables that are not part of the term, the values are neutral values. The neutral value for a quantitative variable is the mean. The neutral value of a nominal variable is the average over all levels which is equivalent to values of 0 for each dummy predictor variable in an effect coding scheme. The neutral value of
an ordinal variable is the reference level.
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