Assume we have a text collection D of 900 documents from three topics (or three classes), Science, Sports, and Politics. Each class has 300 documents. Each document in D is labeled with one of the topics (classes). We use this collection to perform clustering to find three clus- ters. Note that class/topic labels are not used in clustering. After clustering. we want to measure the effectiveness of the clustering algorithm. Cluster Science Sports Politics Entropy Purity 250 20 10 0.589 0.893 2. 20 180 80 1.198 0.643 3. 30 100 210 1.257 0.617 Total 300 300 300 1.031 0.711