Testing archaeological predictive models.

So how does one test an archaeological predictive model or know how well a predictive model works? If you determine how well a model predicts its own input data, what does it tell you about that data or is it just a circular argument? A predictive model of say the locations of individual stores in a chain of stores would give associations with say population densities and transport routes. But what does that tell you about the individual stores, what they sell or the subtle effects of local planning policies, etc? If you have used the archaeological record (HER) or field walking data or archaeological excavation data to make a model, that model will tell you about the location characteristics of that data only. But does that data represent archaeology or past settlement?


Field walking. The results of the Breckland Survey (Suffolk) suggest a correlation between archaeology and artefact scatters. However, the results of the Shapwick Project suggest that the date of material on the surface may not always be a reliable guide to the date of the buried deposits below (see Gerrad. C. ‘Misplaced faith? Medieval pottery and field walking’ in, Medieval Ceramics volume 21, 1997). Further, work carried out in the Rhone valley in France, specifically in relation to archaeological predictive modelling, suggests that field walking results are not always a good indication of what is below the ground surface (see Verhagan. P & Berger. J, ‘The hidden reserve: predictive modelling of buried archaeological sites in the Tricastin-Valdaine region’ in the 28th conference of the CAA, 2001, BAR 931).


Metal detecting. Responsible metal detectorists report their finds to the county archaeological unit. However, it is very rare that they report areas where they have detected but found nothing. In Norfolk (UK) there is a common perception that most metallic archaeological finds are found in the West of the county. Ergo, majority of Norfolk metal deterctorists work in the West of Norfolk, which strengthens this common view. In other words it is a self-fulfilling situation.  


The archaeological record (HER). This tends to be a mixed bag of ad hoc data, subject to various biases such as planning polices (PPG16), the county’s relationship with metal detectorists, the annual budget of the HER officers, etc. As a result it is not easy to model across county or administrative boundaries. The archaeological record is primarily made up of metal detecting, field walking, archaeological excavation, historic buildings and random spot find data.


To produce a good archaeological predictive model, one needs a lot of input data to sample over a small number of categories within any environmental (or social) dataset to obtain sufficient statistical data with which to model with. In other words one cannot sample say 10 pottery sherds over an area with say 15 different soil characteristics and hope to obtain sufficient and reliable data about the relationship between the pottery sherds and the soil! In practice (especially with models that cover a large area) this normally means using the archaeological record (HER) to make the model as there is normally insufficient field walking, metal detecting or archaeological excavation data with which to do so.   


So to reword the primary question; what does one use to test an archaeological predictive model? One important factor is if you are considering all archaeology or just archaeological settlements. As it is difficult to define exactly what constitutes the HER, when modelling all archaeology using the HER, you can only use the HER itself to test the model! This bypasses the need to define exactly what the HER is and what it represents (like individual stores within a chain of stores). For example, remove say a random quarter of the HER, produce a predictive model using the remaining three quarters and then see how well that model predicts the removed quarter.


However, if one is modelling archaeological settlement then the situation is more difficult. Not withstanding the doubts noted above, does a scatter of artefacts found by systematic field walking represent a domestic settlement, a temporary encampment or some sort of industrial activity? Further, domestic settlements can be based on an agricultural economy or a pastoral economy, which require very different environments.


Test pitting on a regular grid system has been quoted as a suitable method to test a predictive model. Another way would be to carry out extensive excavations within the high probability areas of a predictive map to see how well it predicts settlement. Of course a handheld GPS unit and a time machine would be an ideal way of testing a predictive model of archaeological settlement. The important thing is that any archaeological predictive model should be tested by an independent means. Thus, does anyone have any practical suggestions or comments on testing archaeological predictive models?


May 2, 2008 - Posted by | Uncategorized

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