Quantifying the similarity between items is fundamental to neighborhood-based collaborative filtering, yet many commonly used similarity measures are heuristics that lack theoretical justification. The use of heuristics forces practitioners to rely on empirical trial-and-error, offering little insight into why certain measures outperform others or how they might be improved. In this paper, we present a principled approach for designing item similarity measures grounded in probabilistic modeling and parameter estimation. By adopting this formal perspective, we derive two previously overlooked similarity measures based on the Bernoulli and multinomial distributions. Through an extensive ablation study we highlight the critical role of bias, probabilistic smoothing, treating the unknowns correctly, and choosing appropriate probability distributions. Finally, we show that the commonly used cosine similarity can be interpreted as the solution to a specific optimization problem, thereby providing a novel perspective on this decades-old heuristic.

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