A. M. Pollard

Technical note: Regression analysis in adult age estimation

Accurate estimation of human adult age has always been a problem for anthropologists, archaeologists and forensic scientists. The main factor contributing to the difficulties is the high variability of physiological age indicators. However, confounding this variability in many age estimation applications is a systematic tendency for age estimates, regardless of physiological indicator employed, to assign ages which are too high for young individuals, and too low for older individuals. This paper shows that at least part of this error is the inevitable consequence of the statistical procedures used to extract an estimate of age from age indicators, and that the magnitude of the error is inversely related to how well an age indicator is correlated with age. The use of classical calibration over inverse calibration is recommended for age estimation.

Nasty, Brutish, but Not Necessarily Short: A Reconsideration of the Statistical Methods Used to Calculate Age at Death from Adult Human Skeletal and Dental Age Indicators

It is generally assumed that life expectancy in antiquity was considerably shorter than it is now. In the limited number of cases where skeletal or dental age-at-death estimates have been made on adults for whom there are other reliable indications of age, there appears to be a clear systematic trend towards overestimating the age of young adults, and underestimating that of older individuals. We show that this might be a result of the use of regression-based techniques of analysis for converting age indicators into estimated ages. Whilst acknowledging the limitations of most age-at-death indicators in the higher age categories, we show that a Bayesian approach to converting age indicators into estimated age can reduce this trend of underestimation at the older end. We also show that such a Bayesian approach can always do better than regression-based methods in terms of giving a smaller average difference between predicted age and known age, and a smaller average 95-percent confidence interval width of the estimate, Given these observations, we suggest that Bayesian approaches to converting age indicators into age estimates deserve further investigation, In view of the generality and flexibility of the approach, we also suggest that similar algorithms may have a much wider application.

Technical note: Some observations on the conversion of dental enamel δ18op values to δ18ow to determine human mobility

It has become a widespread practice to convert δ(18)O(p) values measured in human and animal dental enamel to a corresponding value of δ(18)O(w) and compare these data with mapped δ(18)O(w) groundwater or meteoric water values to locate the region where the owner of the tooth lived during the formation of the enamel. Because this is a regression procedure, the errors associated with the predicted δ(18)O(w) values will depend critically on the correlation between the comparative data used to perform the regression. By comparing four widely used regression equations we demonstrate that the smallest 95% error is likely to be greater than ±1% in δ(18)O(w) , and could be as large as ±3.5%. These values are significantly higher than those quoted in some of the recent literature, and measurements with errors at the higher end of this range would render many of the published geographical attributions statistically unsupportable. We suggest that the simplest solution to this situation is to make geographical attributions based on the direct comparison of measured values of δ(18)O(p) rather than on predicted values of δ(18)O(w).

Ancient and Modern Specimens of Human Teeth: a Fourier Transform Raman Spectroscopic Study

An assessment is made of the applicability of Fourier transform (FT) Raman spectroscopy to the analysis of ancient human teeth from archaeological sites. Comparisons are made between the Raman spectra of nine modern adult and two exfoliate (deciduous) teeth and those of eight specimens of adult teeth dating from the 4th–16th centuries AD. From the Raman spectra of modern teeth, complete vibrational assignments are proposed for the organic proteinaceous and inorganic carbonated hydroxyapatite components. Band intensity ratio measurements of ancient teeth indicate that the collagen component does not change its conformation with burial age. Several vibrational features arising from specific amino acid residues have been identified, especially proline and hydroxyproline, representing about 10% of the total collagen. Weak bands assigned to citrate have also been assigned. For the inorganic matrix components, quantitative estimations of carbonate content have been carried out. Measurements of the relative intensities of the δ(PO) vibrations at 590 and 430 cm-1 indicate different crystalline orientations in the dental enamel and dentine hydroxyapatite components. Preliminary studies of the phenomenon of ‘pink dentine’ found in ancient teeth from certain burial environments have also been made. It is concluded that FT-Raman spectroscopy can be applied successfully to the analysis of whole ancient teeth or tooth fragments.

Selective biodegradation in hair shafts derived from archaeological, forensic and experimental contexts

Background  Hair is degraded by the action of both dermatophytic and nondermatophytic microorganisms. The importance of understanding hair sample condition in archaeological and forensic investigation highlights the need for a detailed knowledge of the sequence of degradation in samples that have been either buried or left exposed at the ground surface.

Some considerations regarding the use of amino acid racemization in human dentine as an indicator of age at death.

An HPLC method is described for simultaneously obtaining the enantiomeric ratio of three amino acids (aspartic acid, serine, and glutamic acid) from dental collagen, with a view to using this information for estimating age at death. Results are reported from a sample of twenty three known age modern teeth, six known age 19th C. AD teeth, and two unknown age Romano-British teeth. It was found (as expected) that all three D/L ratios changed significantly with chronological age. Standard calibration techniques were used to estimate ages for the six 19th C. AD specimens from regression equations estimated from the modern specimens, and also to predict (for the first time) the error associated with such estimates. Errors using aspartic acid were found to be similar to those obtained by other methods of age estimation from dental evidence, serine, and glutamic acid providing much poorer age estimates. Additionally, a systematic difference in the age-enantiomeric ratio relationship was observed between modern and older dental samples. It is concluded that there is some fundamental difference in the observed enantiomeric ratios between modern teeth and older samples, possibly as a result of the chemical alteration of the dental proteins.

Nonparametric calibration for age estimation

A method is proposed for the calibration of a continuous random variable when the dependent variables are a combination of continuous and categorical, and the model between the controlling variables and calibrated variable is empirically derived. The various probability distributions are estimated from training data by using kernel density procedures with bi-variate normal kernels for continuous variables and uniform smoothing for discrete variables. Bayess theorem is then used to produce the posterior distribution from which point estimates and estimates of confidence may be made. Individual posterior densities allow each case to be considered separately and cases with conflicting evidence can easily be identified for further investigation. This approach is illustrated by using part of a data set of human adult teeth from individuals of known age. Estimates from the method proposed show less bias than those from the widely used multiple regression. This allows a more accurate reconstruction of the age distributions of ancient populations. In particular bias reduction is most notable at the extreme ages, which also tend to be the least frequent, thereby widening the age distribution. This will allow a more reliable consideration of archaeological and anthropological questions relating to, for example, the maximum lifespan, age-related social structure and the development of age-related disease. Copyright 2002 The Royal Statistical Society.

FURTHER COMMENTS ON THE ESTIMATION OF ERROR ASSOCIATED WITH THE GUSTAFSON DENTAL AGE ESTIMATION METHOD

Many researchers in the field of forensic odontology have questioned the error estimates stated in Gustafsons paper outlining the relationship between certain dental attributes and age. In a substantial re-working of Gustafsons data, Maples and Rice corrected Gustafsons regression statistics and found that the error associated with the age estimate was nearly double that claimed by Gustafson. We offer another statistical analysis of Gustafsons data and find that the errors calculated by Maples and Rice were also in error, being about a year too small. We give a formula for what we believe to be the correct treatment of errors in such cases, but conclude by observing that there is an urgent need for a more rigorous study of the traits first tabulated by Gustafson.

Radiocarbon calibration and Late Glacial occupation in northwest Europe

Various methods of analysing the dating of the late Glacial suggest various interpretations. Here, in answer to a paper from 1997, radiocarbon dates are calibrated and used to reconsider the dating of this contentious period.

A new interpretative approach to the chemistry of copper-alloy objects: source, recycling and technology

The metal composition of bronze alloys has been routinely examined as a means of inferring the source of the ore. But bronze is recycled, and the quantity of some components, such as arsenic, is depleted every time the alloy is melted down. Since the Early Bronze Age of the British Isles was largely supplied from a single mine on Ross Island, Co. Kerry, tracking arsenic content shows the number of re-melts and this gives the object a biography and a social context. Applying this ingenious new procedure to their large database, the authors also winkle out other sources of supply and new insights about the technology involved.