A necessary and sufficient stability notion for adaptive generalization

We introduce a new notion of the stability of computations, which holds under post-processing and adaptive composition, and show that the notion is both necessary and sufficient to ensure generalization in the face of adaptivity, for any computations that respond to bounded-sensitivity linear queries while providing accuracy with respect to the data sample set. The stability notion is based on quantifying the effect of observing a computation's outputs on the posterior over the data sample elements. We show a separation between this stability notion and previously studied notions.