Definition
A population report (or "pop report") is a publicly searchable database maintained by each major grading service. It tracks how many copies of every graded item exist at each grade level. Pop reports are central to pricing modern and vintage graded items because they quantify scarcity at the top of the scale.
Why it matters
Population scarcity at high grades creates pricing tiers. A card with a tiny top-grade population can sell for many multiples of an otherwise identical card with a large top-grade population.
Example
A 2019 Topps Update Yordan Alvarez rookie has a PSA 10 population of 200 and a PSA 9 population of 1,800. Buyers price the 10 at a significant premium because of the relative scarcity.