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How long were they used? Durations of activity at five Neolithic long barrows Right A key problem with ‘fuzzy’ dates is that we end up confusing the duration of past activity with our ignorance of the actual date. The new dating programme provides realistic estimates for the durations of specific activities in the prehistoric past. It turns out that burial at the five long barrows was a matter of generations, not centuries as previously believed. C H A R T: E N G L I S H H E R I T A G E only been able to assign the people whom they study to imprecise times. As a result, prehistory is often seen as a fuzzy period, a timeless stretch in which nothing changes for long periods. The new dating system suddenly means that prehistorians can work within the time-scales which Romanists or Anglo-Saxonists expect. We can start thinking about the Neolithic in terms of individuals and communities. We can begin to create Neolithic histories – ideas, events and people at specific times over 5,000 years ago.' How is this possible? It is not a result of new excavations, but rather the culmination of a decade of multi-disciplinary research which has until now focused largely on specific sites such as Skara Brae, Stonehenge and Hambledon Hill. 'This,' says Bayliss, 'is archive archaeology in action.' In research funded by a partnership of English Heritage, the Arts and Humanities Research Council, and the Leverhulme Trust, a total of about 1500 radiocarbon dates from Early Neolithic sites in Southern Britain have been examined – 169 from five long barrows, 748 from 39 causewayed enclosures, and 600+ from a variety of other sites that can confidently be assigned to the period. The research has been undertaken by English Heritage and Cardiff University, in conjunction with the Universities of Central Lancashire, Oxford, and Groningen in The Netherlands, with help from many excavators, units, and museums. The first phase of the new dating programme – for the five long barrows – is now complete. The result is an academic bomb-shell that reveals the extraordinary potential of a new approach to dating which combines Bayesian statistical methods and the processing power of modern computers. The mathematics – embodied in a theorem first published in 1764 – are complex (see box 'Bayesian Statistics for Beginners'). The crux of the matter is that the date range represented by an individual radiocarbon determination can be compressed if we take account of other information – about context, sequence, sample character, and other radiocarbon determinations from the site. To some degree archaeologists have always done this. Radiocarbon determinations from different levels of a site are compared and guesstimates made of actual dates which are narrower than those of the laboratories for individual samples. 'The problem,' explains Bayliss, 'is that this approach has no mechanism to allow for the statistical scatter on the radiocarbon dates, so it looks as if archaeological activity started earlier, continued for longer, and ended later than was actually the case. In effect, the scatter and uncertainties on the radiocarbon dates are being confused with longevity of ancient activity.' A simple statistical approach – averaging probabilities from a series of radiocarbon determinations – cannot solve the problem. 'This method misleads us into believing that the archaeological activity continued for much longer than it did in reality. Indeed, the more dates are included in the analysis, the more scattered will be the results and the longer the estimates for the duration of the activity.' Bayesian statistics represent a much more sophisticated form of analysis. 'What we're doing is putting the Harris matrix – the whole stratigraphic sequence on the site – into the equation,' says Bayliss. 'And we have to insist on the integrity of the samples. I'm a complete fascist about this. Bayesian models are just like other computer models – junk in, junk out. So it's essential to exclude all residual and intrusive material. Ideally we need radiocarbon dates from articulated bone.' The point about articulated bone is that 209 archaeologycurrent 15 209

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