SUPPLEMENTARY MATERIAL: Pairwise Similarity Learning for Chronological Attribution of Archaeological Assemblages: A Siamese Neural Network Approach
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Abstract
We present a similarity-based framework for the chronological attribution of undated archaeological assemblages, grounded in Siamese Neural Networks (SNNs) and specifically designed for the small sample sizes, compositional uncertainty, and diffuse phase boundaries that characterize most prehistoric datasets. Unlike conventional supervised classifiers, which require sufficient per-class examples to define stable decision boundaries, SNNs reformulate chronological inference as a pairwise relational problem: the model estimates the probability that two assemblages belong to the same chronological phase, mimicking the comparative reasoning implicit in typological analysis while rendering it reproducible and transferable. The quadratic growth of training pairs with sample size substantially amplifies the effective training set without additional radiocarbon data, a critical advantage in data-scarce contexts.
The framework is evaluated on 185 radiocarbon-dated bifacial flint arrowhead assemblages from eastern Iberia (ca. 3500–1900 cal. BC), organized into six chronological phases spanning the Late Neolithic to the Early Bronze Age. Multiple Siamese configurations — logistic regression, MLP, random forest, and deep learning encoders — are compared against standard MLP and SVM baselines. The best-performing configuration (DL with bootstrap augmentation and Dirichlet-multinomial compositional variables) achieves a macro F1 of 21.4% and balanced accuracy of 21.8%, representing a consistent improvement over both baselines and over a stratified random classifier (expected macro F1: ~17%) across all class-balanced metrics. Per-type analysis reveals that predictive accuracy correlates with morphological distinctiveness: foliaceous types reach 96.5% agreement with expert assignments, while pedunculate forms — whose typological boundaries are inherently contested — present the greatest classification challenge, a pattern that mirrors the gradient of archaeological interpretive confidence.
The input structure of the framework — assemblage-level frequency vectors of artifact types — is standard across the main material categories used for chronological inference in prehistoric archaeology, including ceramics, lithic industries, and faunal assemblages. The approach is therefore directly transferable to other material traditions and regional sequences, and its outputs are formally compatible with Bayesian chronological modelling pipelines. The framework requires only typological frequency data routinely collected during excavation and post-excavation analysis, involves negligible computational cost relative to radiometric methods, and can be deployed at early stages of site investigation to generate provisional chronological attributions — providing an operational basis for decisions about sampling strategy, resource allocation, and the prioritization of contexts for absolute dating. As radiocarbon coverage and regional reference databases expand, the framework scales accordingly, functioning as a cost-effective first-pass instrument within a broader chronological workflow rather than as a replacement for higher-resolution analytical methods.
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aposteriori_predictions_wide.csv
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Additional details
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- Programming language
- Python