Today was the first meeting of the Gaia Unlimited project (PI: Anthony Brown), in which we attempt to make a selection function (and the tools for making many different kinds of selection functions) for investigators making use of the ESA Gaia data to perform population-level inferences. Among the many things we discussed were the definition of the selection function (which is not trivial, given the historical usage of the term (and it appears in 700 refereed publications in 2020, according to NASA ADS), and what's known about the Gaia selection function already. The latter includes amazing work by Boubert and Everall in which they have tried to reverse engineer everything to determine the selection function to very faint levels, given the Gaia scan patterns, telemetry limits, and dropped fields. So far, my role in this project is on the conceptual side, around definitions, terminology, and use cases. Along those lines there was great discussion about what the selection function is, and what it is not. Our position is that it is the probability, given hypothetical properties q, that a counter-factual source with those properties would enter the catalog. Even that definition is not quite complete, because there are details relating to the observability of—and noise in—the properties q. More about this throughout this year!
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