Internet Archaeol. 17. Willis. The subject group to be 'dated'

5.3.2.2 The subject group to be 'dated'

For the method to work, several prerequisites are needed in terms of the site group (the subject group) that one may wish to 'date' using the database.

The subject group will need to be of some size and contain sufficient vessels of particular representative forms in order to establish a ratio; hence several examples of forms will need to be present. If the sample is fairly large this may improve chances of accurate and reliable dating, depending upon other factors.
The subject group will need to be reasonably discrete chronologically; that is not extend for more than 100 years, and preferably much less, ideally not more than 50 years. An amount of residual samian in the group should not be problematic, unless it forms a large proportion of the group, in which case this should be apparent from various indicators.
Groups with an unusual composition, such as structured deposits, are unlikely to be suitable for dating in this way. (An exception may be the group from the annex ditch at Tullie House, Carlisle (see 5.3.2.4).)
The method may be more reliable for some periods as opposed to others.
The form types among the subject group that are to be selected for study need to be chosen with a little care. Appropriate selection depends on the broad date of the group/phase. Some possibilities are suggested here (Table 4 below). The pairings in Table 4 relate to forms that are either successive or which overlap in currency. Some of these pairings have been considered from the point of view of establishing chronology in previous work (e.g. Groves 1993; Tyers 1993, 137-8).

Suspected broad date of group:	'Good' chronological indicators
Later 1st century AD	Drag.15/17 platter v Drag. 18 platter: the higher the proportion of the latter the later the group
Later 1st century AD	Drag. 24/25 cup v Drag. 27 cup: the higher the proportion of the latter the later the group
Later 1st century AD	Drag. 29 bowl v Drag. 37 bowl: the higher the proportion of the latter the later the group
Mid 2nd century AD	Drag. 27 cup v Drag. 33 cup: the higher the proportion of the latter the later the group
2nd century AD	Drag. 18/31 or 18/31R dishes v Drag. 31 (dish) or 31R (bowl): the higher the proportion of the latter, the later the group, post c. AD 150/160
Later second century AD	Drag. 31 (dish) v 31R (bowl): the higher the proportion of the latter, the later the group, post c. AD 160

Table 4: Ratios between samian form types within groups: some potential indicators of broad chronology

The date-range suggested by reference to the database will, of course, in turn need to be considered and weighed alongside the dates of the coarse pottery and other indices of date in post-excavation studies.