Defect clustering
Beginner
Flaws on a wafer tend to bunch together instead of landing evenly, so some chips get several and others none.
Novice
Killer defects are not spread perfectly at random: they gather in some regions, wafers or lots. For a given average defect density, clustering leaves more dies defect-free than a purely random spread would.
Expert
Modeled by letting the defect density itself vary (compound Poisson). A gamma distribution gives the negative binomial model with cluster parameter ; small means strong clustering, and of about 10 or more behaves like Poisson.