Determining the Final Grouping of Clusters
main topic      

The final grouping of clusters (also called the final partition) is the grouping of clusters which will, hopefully, identify groups whose observations or variables share common characteristics. The decision about final grouping is also called cutting the dendrogram. The complete dendrogram (tree diagram) is a graphical depiction of the amalgamation of observations or variables into one cluster. Cutting the dendrogram is akin to drawing a line across the dendrogram to specify the final grouping.

How do you know where to cut the dendrogram? You might first execute cluster analysis without specifying a final partition. Examine the similarity and distance levels in the Session window results and in the dendrogram. You can view the similarity levels by placing your mouse pointer over a horizontal line in the dendrogram. The similarity level at any step is the percent of the minimum distance at that step relative to the maximum inter-observation distance in the data. The pattern of how similarity or distance values change from step to step can help you to choose the final grouping. The step where the values change abruptly may identify a good point for cutting the dendrogram, if this makes sense for your data.

After choosing where you wish to make your partition, rerun the clustering procedure, using either Number of clusters or Similarity level to give you either a set number of groups or a similarity level for cutting the dendrogram. Examine the resulting clusters in the final partition to see if the grouping seems logical. Looking at dendrograms for different final groupings can also help you to decide which one makes the most sense for your data.