Figure 2.

Schematic of silhouette widths, i.e. homogeneity scores, and silhouette plots. A. (left) Given an element in a group (the orange cross surrounded by a diamond) the distances from elements in the same group (magenta lines) and from those in the other group (green lines) are computed. The homogeneity score can be viewed as the difference between the averages of the inter-group distance (green) and the intra-group distance (magenta). (right) The homogeneity score of each sample is plotted on a horizontal bar, after sorting the samples within each group. The average of the homogeneity scores is computed for each group yielding an estimation of the homogeneity of the cluster. B. Four different categories of homogeneity (left) and the corresponding silhouette plots (right) are depicted. Specifically: Scenario 1. two homogeneous and well-separated groups; Scenario 2. one homogeneous and one heterogeneous group, well-separated; Scenario 3. one homogeneous and one heterogeneous group, overlapping; Scenario 4. two heterogeneous overlapping groups. The empirical interpretation of the average homogeneity score for a group is shown at the bottom.

Sboner et al. BMC Medical Genomics 2010 3:8   doi:10.1186/1755-8794-3-8
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