Occurrence seriation is the ordering or arrangement of items (artifacts or assemblages) according to the presence or absence of their constituent features (practitioners of occurrence seriation often refer to a "class" or "style" what OptiPath considers a feature). The seriation Technique can be set in the Seriations table.
The underlying assumption in archaeological occurrence seriation is that a feature will normally be introduced only once in the evolution of an artifact type - once the feature has appeared and then been absent in artifacts for a period of time it will not reappear. Most implementations of occurrence seriation attempt to order the artifacts temporally so as to minimize the number of times that a distinct feature is introduced into the archaeological record. Equivalently, occurrence seriation tries to minimize the total number of transitions (from absent to present, and present to absent) for all features.
Occurrence seriation is a special case of the Optimal Path seriation model. Setting the seriation parameter Technique to Occurrence specifies OptiPath's standard implementation of occurrence seriation. Data may take on any value but zeroes and blanks are both assumed to represent an absence of the feature (class or style) in question. All other values are taken to indicate the presence of the feature. Setting the seriation parameter Technique to Occurrence 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 occurrence seriation. To change them you must first select the Custom Seriation technique.
| Parameter | Value |
| Data | Classed |
| Ranks | 1 |
| Metric | Hamming |
| Normalize | No |
| Transition | 0.5 |
| Earlier | Absent & Unknown |
| Later | Absent & Unknown |
| Blanks | Absent & Unknown |
| Zeroes | Absent & Unknown |
These settings result in any non-zero entry in the Data being interpreted as indicating the presence of a feature (class or style) and any blank or zero in the data being interpreted as the absence. If you do not want blanks or zeroes to be interpreted in this way, you should set the seriation parameter Technique to Custom and the feature parameters Blanks and Zeroes. To see the effect of your settings, look at processed data in the Values table.
With the settings above, if your data consists entirely of zeroes and ones, the zeroes will be interpreted as absent and the ones as present. As another example, the data for a feature might contain the values "3", "7", "notched", "palmate", "serrated" and "lobed", as well as blanks and zeroes. In this case the blanks and zeroes would be interpreted as absent and all other entries as present. To see the effect of your settings, look at processed data in the Values table.
Warning! If your data are not numbers but alphanumeric values still representing absence and presence such as ("absent" and "present" or "no" and "yes" or "zero" and "one"), the results will not be the same. OptiPath will interpret "absent" as present. The advantage of using zero/one data over "absent"/"present" is that OptiPath can recognize zeroes and treat them as absences (via the Zeroes parameter) but OptiPath has no facility to recognize words like "absent" or "no" or "zero" and treat them specially. For more, see Data.
In occurrence seriation, careful thought should be put into setting the feature parameters appropriately - see Setting the Earlier, Later, Blanks, Zeroes and Transition Parameters.