Determine the final grouping of clusters

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. The following steps can help you determine where to cut the dendrogram.

  1. Perform a cluster analysis using the default setting (1 cluster in the final partition). Minitab displays the results for all possible numbers of clusters.
  2. Examine the similarity and distance levels in the Session window results and in the dendrogram. You can view the similarity levels by putting your mouse pointer over a horizontal line in the dendrogram. The similarity level at any step is the percentage of the minimum distance at that step relative to the maximum inter-observation distance in the data.
  3. Choose where you want to make the final partition. The pattern of how similarity or distance values change from step to step can help you choose the final grouping. The step where the values change suddenly might identify a good point for cutting the dendrogram, if this is logical for your data.
  4. Perform the clustering procedure again, 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.
  5. Examine the resulting clusters in the final partition to determine whether the grouping seems logical. Examining dendrograms for different final groupings can also help you decide which one is the most logical for your data.

For some data sets, average, centroid, median and Ward's methods might not produce a hierarchical dendrogram. That is, the amalgamation distances do not always increase with each step. In the dendrogram, such a step will produce a join that goes downward instead of upward.

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