Methods to handle missing values are:
• Listwise deletion excludes a case from the analysis, if one or more variables are missing.
If many variables are used to cluster cases, the number of cases may be reduced
dramatically.
• Pairwise deletion uses all available information. A case is only eliminated, if the number
of missing values exceeds a certain threshold.
• Estimating missing values with imputation techniques (see Rubin 1987, Little and Rubin
1987, Gordon 1999: 26-28).
Table 3-2 shows an example. Case g has a missing value in X4, case g* in X3. Both cases
would be eliminated by listwise deletion of cases. In contrast to this, pairwise deletion of
values would use the Variables X1, X2 and X5 and compute a mean or re-scaled similarity or
dissimilarity measure using the following formula: