An optional logical flag indicating whether the X's used to estimate the "middle" matrix should be chosen in a pairwise fashion or casewise fashion. If pairwise, the correlation between observations $i$ and $j$ is based on the time periods common to $i$ and $j$. If casewise, the correlation between observations i and j is based on the largest rectangular subset of the data, i.e., $T_i$ = $T_j$ = $T^*$ for all $i$ and $j$ if casewise is selected.