An estimate of the fraction of trades that come from informed traders, derived from imbalances in buy- and sell-initiated order flow over time. Formally, in the Easley–Kiefer–O'Hara–Paperman (1996) model, PIN = αμ / (αμ + varepsilon_b + varepsilon_s), where α is the probability of an information event, μ the informed-trade arrival rate, and varepsilon_b, varepsilon_s the uninformed buy and sell arrival rates.