 That algorithm, however, requires knowing in advance the number of clusters that generated the data.In many situations it is not possible to determine this ahead of time, and even when we can reasonably assume a number of clusters we would still like to be able to check this assumption.For example, in the voting example above the division into liberal, conservative and moderate might not be finely tuned enough; attributes such as a religion, class or race could also be critical for modeling voter behavior, resulting in more clusters in the model.