Typical objective functions in clustering formalize the goal of attaining high intra-cluster similarity (documents within a cluster are similar) and low inter-cluster similarity (documents from different clusters are dissimilar). This is an internal criterion for the quality of a clustering. But good scores on an internal criterion do not necessarily translate into good effectiveness in an application. An alternative to internal criteria is direct evaluation in the application of interest. For search result clustering, we may want to measure the time it takes users to find an answer with different clustering algorithms. This is the most direct evaluation, but it is expensive, especially if large user studies are necessary.
常用內部（Sil，CH，DBI，KL）、外部評價指標（Rand等4個）
1. 蘭德指數
常用內部（Sil，CH，DBI，KL）、外部評價指標（Rand等4個）
1. 蘭德指數
