Nominal seriation is a generalization of Occurrence seriation. Instead of categorizing each feature into two classes (present or absent, red or blue), the technique allows any number of classes. If the classes are numbered, it does not imply that items should be seriated in order of their class number (see Classed Data). The seriation technique can be set in the Seriations table. Nominal seriation is normally used when only the presence or absence of feature classes is known and metric data is unavailable.
Nominal seriation is a special case of the Optimal Path seriation model. Setting the parameter Technique to Nominal in the Seriations table specifies OptiPath's standard implementation of nominal seriation. Setting the seriation parameter Technique to Nominal causes OptiPath to automatically set the Feature parameters Data, Ranks, Metric, Normalize, Transition, Earlier, Later, Blanks and Zeroes to specific values.
The following are the default Feature parameter settings for Nominal seriation. To change them you must first select the Custom seriation technique.
| Parameter | Value |
| Data | Classed |
| Ranks | 0 |
| Metric | Hamming |
| Normalize | No |
| Transition | 10 |
| Earlier | Absent & Unknown |
| Later | Absent & Unknown |
| Blanks | Absent & Unknown |
| Zeroes | Present & Zero |
These settings result in any non-blank entry in the Data being interpreted as indicating the presence of a feature (class or style). If you do not want blanks or zeroes to be interpreted in this way, you should set the seriation parameter Technique to Custom and reset the feature parameters Blanks and Zeroes. To see the effect of your settings, look at processed data in the Values table.
In nominal seriation, careful thought should be put into setting the feature parameters Earlier and Later, Blanks and Zeroes appropriately - see Setting the Earlier, Later, Blanks, Zeroes and Transition Parameters.