For {displaystyle s(i)} to be close to 1 we require {displaystyle a(i)ll b(i)} . As {displaystyle a(i)} is a measure of how dissimilar {displaystyle i} is to its own cluster, a small value means it is well matched. Furthermore, a large {displaystyle b(i)} implies that {displaystyle i} is badly matched to its neighbouring cluster. Thus an {displaystyle s(i)} close to one means that the datum is appropriately clustered. If {displaystyle s(i)} is close to negative one, then by the same logic we see that {displaystyle i} would be more appropriate if it was clustered in its neighbouring cluster. An {displaystyle s(i)} near zero means that the datum is on the border of two natural clusters.
The average {displaystyle s(i)} over all data of a cluster is a measure of how tightly grouped all the data in the cluster are. Thus the average {displaystyle s(i)} over all data of the entire dataset is a measure of how appropriately the data has been clustered. If there are too many or too few clusters, as may occur when a poor choice of {displaystyle k} is used in the clustering algorithm (e.g.: k-means), some of the clusters will typically display much narrower silhouettes than the rest. Thus silhouette plots and averages may be used to determine the natural number of clusters within a dataset. One can also increase the likelihood of the silhouette being maximized at the correct number of clusters by re-scaling the data using feature weights that are cluster specific