A key decision the facilitator must make is to determine the number of clusters to be used to divide the map space for the Interpret Maps step.

There is no rule that can accomplish this automatically--the facilitator must use his/her judgment. Essentially, the analyst uses discretion in examining different cluster solutions to decide on the one that makes sense for the case at hand. While there are many ways this might be accomplished, we have developed a general procedure that we usually follow to facilitate this decision.

First, the facilitator determines the range of numbers of clusters that might be reasonable for a particular project. For instance, in a project with 100 statements, it is reasonable to expect that the number of clusters should be greater than 5 but fewer than 20. Why? Because there should be enough categories (i.e., more than 5) to be useful but not so many categories (i.e., more than 20) that some of them would have only one or two statements in them.

Once the range is established, the facilitator should examine the different cluster solutions in that range to see which one seems to fit best and be most potentially useful. To do this, he/she examines the cluster solutions in reverse order (i.e., from 20 down to 5). At each step, the analysis is merging two clusters from the previous step. For instance, when moving from the 20 to 19 cluster solution, the analysis merges two clusters. At each step, the facilitator examines the clusters that were merged and tries to make a subjective determination about that merger. Sometimes the statements from the two merged clusters seem to fit together well. At other times, the merger seems to obscure an important distinction that the separate clusters preserved. It usually becomes apparent that beyond some number of clusters (e.g., solutions with less than 14 clusters), distinctions are lost that should be preserved. Generally, the facilitator is then able to make a judgment about the cluster solution that yields the fewest number of clusters but still preserves the maximum number of interpretable categories, and that is the solution to be selected as the input for the interpretation session.

Once the number of clusters has been decided, the facilitator needs to prepare interpretation materials. How this is done depends on whether the interpretation session will be done manually (i.e., with maps and listings printed on paper) or on screen. See the Interpret Maps step for more information on preparing for and conducting the interpretation session.

It is important to keep in mind that the initial number of clusters is not set in stone. It is only a best-guess estimate and can be changed during the course of the interpretation. Usually the facilitator makes the decision alone; but in some situations, it is also possible to do this through a discussion with a small group of key stakeholders (e.g., the initiator and two or three key participants).