We introduce a simple measure of informed trading risk, , the residual to liquidity quote‐improvement‐to‐deterioration ratio times . When facing with increased informed trading risk, liquidity providers compete less to provide liquidity, reducing their undercutting activity. Reductions in undercutting leave footprints in trade and quote data that are captured by . Unlike prior measures, is easy to construct, can be computed intraday, and is orthogonal to liquidity. The measure outperforms prominent existing alternatives in reflecting the extent of information asymmetry before earnings announcements, predicting unscheduled press releases, and identifying informed trading spillovers around them.

Tick sizes affect market quality through a tradeoff between pricing fidelity and undercutting. The U.S. Tick Size Pilot (TSP), which raised the minimum tick from 1¢ to 5¢, provides a natural experiment to study this tradeoff. We find that the TSP harmed liquidity for stocks with spreads below 10¢ but improved liquidity for stocks with spreads above 15¢. These opposing effects explain the mixed results across prior studies which pool together stocks with very different prevailing spreads. We recommend researchers using the TSP for causal inference should, at minimum, split samples at 10¢-spreads to account for these heterogeneous liquidity effects.