Method assessment and observer variation in age estimation: A comparative analysis of the Suchey-Brooks and the Iscan methods on an archaeological medieval population

Age-at-death estimation methods have faced challenges in their applicability to past populations. This study aims to evaluate the assessment and observer variability of the Suchey-Brooks (SB) and Iscan methods within archaeological contexts. Our sample consisted of 400 individuals from the medieval cemetery of the Dunes Abbey in Koksijde, Belgium. The skeletal remains exhibited good preservation, especially for the Iscan can methods, which showed higher sensitivity to preservation conditions. Intra- and inter-observer variation were investigated using Spearman correlation coefficients, Wilcoxon paired ranks test, and Cohen's kappa test. The Spearman coefficient revealed a strong positive correlation, with a moderate to almost excellent kappa coefficient for both intra- and inter-observer error. Overall, the SB method displayed better inter-observer agreement, while the Iscan method presented better intra-observer agreement. Applying these methods to younger individuals resulted in less variability. Certain descriptive phase categories, such as phase III for the SB method and phase 3 and 4 for the Iscan methods, seemed problematic. Only 188 sufficiently preserved individuals could be scored using both the SB and the Iscan can methods. They were classified into age groups, and the methods assessment were evaluated using the same statistical approach. The Spearman correlation coefficient indicated consistent age group attribution between the two methods (rho = 0.92), with a moderate to almost excellent agreement (k = 0.45; wk = 0.91). Overall, the two methods mainly differ by only one age group, except for older adults. The SB method tended to assign older age groups compared to Iscan's method, leading to effective overestimation.