silhouette calculates and graphs the silhouette width for the cluster solution given by the grouping variable, using the pairwise distance matrix given in the distmat option. Silhouette width is an indicator of cluster adequacy. It compares for each case, the mean distance to other cases in the cluster in which the case is, and the mean distance to the nearest neighbour cluster. Silhouette widths less than zero indicate a case that fits poorly in its cluster.