Gordon formulates additional criteria: A cluster should contain similar objects and should
satisfy additional criteria. Compared to Everitt, Gordon’s definition portrays the development
in cluster analysis. Until the 80s the discussion concentrated mainly on techniques. At the end
of the 80s the whole process of clustering starting with the selection of cases and variables
and ending with the validation of clusters became dominant. The steps in a clustering
process are:
1. selection of appropriate cases, variables and methods
2. application of the methods
3. evaluation of the results.
This last step includes:
1. determination of the number of clusters, if unknown
2. substantive interpretation of clusters
3. test of stability
4. test of internal validity (model fit), relative validity and external validity