Extends the ModelMatrixPlus class
ZLogical indicating treatment assignment
StrataFrameFactors indicating strata
ClusterFactor indicating who's in the same cluster with who
If the DesignOptions represents clusters of elements, as when it was created by aggregating another DesignOptions or ModelMatrixPlus object, then its Covariates and NotMissing slots are populated with (weighted) averages, not totals. E.g., NotMissing columns consist of weighted averages of element-wise non-missingness indicators over clusters, with weights given by (the element-level precursor to) the UnitWeights vector. As otherwise, columns of the NotMissing matrix represent terms from a model formula, rather than columns the terms may have expanded to.
If present, the null stratification (all units in same stratum) in indicated
by the corresponding column of the StrataFrame slot bearing the name
‘--’.