Bias in the PAS Database: The Case of Annular BroochesPosted: May 18, 2013
Over the course of my research into fifth- and sixth-century Anglo-Saxon brooches, I’ve noticed some substantial disparities between metal-detected and archaeological sources of data. As the Portable Antiquities Scheme becomes an increasingly popular mode of studying the past (the PAS website currently lists 366 projects), we would do well to study the means by which these data are produced and their inherent biases.
The most signficant bias I have come across in the PAS database is its massive over-representation of cruciform brooches compared to the pitiful number of annular brooches it has recorded. Cruciform brooches and annular brooches make a good comparison because they both have the same distribution in eastern England. The vast majority of both archaeologically excavated and metal-detected brooches are also likely to originate from the same contexts: graves, disturbed or otherwise.
A simple search on the PAS database reveals 686 examples of cruciform brooch (plus a further 78 fragments that are likely to have originated from cruciform brooches, but which lack diagnostic features) and 101 annular brooches (including a handful that might be of a slightly later 7th century date). I’ve provided detail on the searches I used at the bottom of the post, so that these results can be repeated and verified.
I compared these results from a sample of nine excavated and published cemeteries from eastern England: Castledyke South and Fonaby from Lincolnshire; Morning Thorpe, Westgarth Gardens and Bergh Apton from East Anglia; Empingham II and Broughton Lodge from the East Midlands; Norton and Sewerby from the Yorkshire and the North. The excavated cemeteries produced 390 annular brooches and only 147 cruciform brooches. As can be seen from the histogram (Figure 2), the proportions of these brooches are virtually reversed depending on whether they were excavated or metal-detected.
This massive difference might be partly explained by an over-representation of cruciform brooches: they are larger objects and tend to break up into more pieces in the plough soil. Being more substantial, they might also produce a stronger signal to a metal detector. However, even these factors cannot account for this degree of bias: much smaller items than fragments of annular brooch are frequently found on the PAS, including cruciform brooch knobs. Additionally, annular brooches frequently break up into several pieces too, albeit less substantial ones. The most promising explanation in my mind is that annular brooches are actually found far more regularly than the PAS database suggests, but they are only rarely reported to Finds Liaison Officers. Annular brooches are distinctly plain objects and might easily be mistaken for a piece of bronze washer. I imagine though, given the knowledge and experience of most metal-detectorists, that these object are actually regularly recognised, but just not deemed worthy of reporting given their dull appearance.
For the early Anglo-Saxon period, such biases are not devastating because we can check the excavated and metal-detected data against each other. Nevertheless, this very basic analysis reveals the severe risks of working only from metal-detected data. The impact of different taphonomic processes and finds reporting practices is considerable. Research based on just one of these sources would produce the opposite findings.
Please do comment below if you have seen similar biases in other material. Any alternative explanations of this problem from either the perspective of the PAS, metal-detectorists or archaeologists are also most welcome.
Portable Antiquities Scheme Data Collection
The search was done using only finds whose records are publicly available. Search terms were as follows:
- Object type: “BROOCH”
- Object description contains: “cruciform” / “annular”
- Broad period: “EARLY MEDIEVAL”
- Sub-period from: “early”
- Sub-period to: “early”
The search results were then limited to just objects with accompanying images, so that the identification of the object could be checked. Each sample was then scanned through manually to remove any stray items that also contained the term “cruciform” or “annular” in their object descriptions. These totals represent substantially less than is actually present in the online database, but as the same standard search terms were used to obtain both samples, the data is unlikely to be skewed at this level. Biases in PAS search terms is a subject I’m going to address in a future post.
- Cook, A. M. (1981) The Anglo-Saxon Cemetery at Fonaby, Lincolnshire. Sleaford: The Society for Lincolnshire History and Archaeology.
- Drinkall, G. and Foreman, M. (1998) The Anglo-Saxon Cemetery at Castledyke South, Barton-on-Humber. Sheffield: Sheffield Academic Press.
- Green, B. and Rogerson, A. (1978) The Anglo-Saxon Cemetery at Bergh Apton, Norfolk. Gressenhall: The Norfolk Archaeological Unit.
- Green, B., Rogerson, A. and White, S. G. (1987) The Anglo-Saxon Cemetery at Morning Thorpe, Norfolk. Gressenhall: The Norfolk Archaeological Unit.
- Hirst, S. (1985) An Anglo-Saxon Saxon Inhumation Cemetery at Sewerby East Yorkshire. York: York University Archaeological Publications.
- Kinsley, A. G. (1993) Broughton Lodge: Excavations on the Romano-British Settlement and Anglo-Saxon Cemetery at Broughton Lodge, Willoughby-on-the-Wolds, Nottinghamshire, 1964-8. Nottingham: University of Nottingham, Department of Classical and Archaeological Studies.
- Sherlock, S. J. and Welch, M. G. (1992) The Anglo-Saxon Cemetery at Norton, Cleveland. London: Council for British Archaeology.
- Timby, J. R. (1996) The Anglo-Saxon Cemetery at Empingham II, Rutland. Oxford: Oxbow.
- West, S. E. (1988) Westgarth Gardens Anglo-Saxon Cemetery Suffolk: Catalogue. Bury St Edmunds: Suffolk County Planning Department.