Knowing when to stop: insights from ecology for building catalogues, collections, and corpora

A major locus of musicological activity-increasingly in the digital domain-is the cataloguing of sources, which requires large-scale and long-lasting research collaborations. Yet, the databases aiming at covering and representing musical repertoires are never quite complete, and scholars must contend with the question: how much are we still missing? This question structurally resembles the 'unseen species' problem in ecology, where the true number of species must be estimated from limited observations. In this case study, we apply for the first time the common Chao1 estimator to music, specifically to Gregorian chant. We find that, overall, upper bounds for repertoire coverage of the major chant genres range between 50 and 80 %. As expected, we find that Mass Propers are covered better than the Divine Office, though not overwhelmingly so. However, the accumulation curve suggests that those bounds are not tight: a stable ~5% of chants in sources indexed between 1993 and 2020 was new, so diminishing returns in terms of repertoire diversity are not yet to be expected. Our study demonstrates that these questions can be addressed empirically to inform musicological data-gathering, showing the potential of unseen species models in musicology.