What is Delisting?
Delisting is when a stock stops trading on an exchange. It can happen for neutral reasons, such as a takeover, or bad ones, such as bankruptcy or falling below the exchange's listing rules. A delisted stock's final price move is often a large loss, and data that drops delisted names loses that move.
Why Delisting matters
Leaving delisted stocks out of a test removes many of the worst outcomes and flatters the result.