MDF
Minimum Defense Frequency — the minimum defending frequency against a bet
Definition
MDF (Minimum Defense Frequency) is the minimum share of your range you must continue with (call or raise) against a bet so you can't be exploited by automatically profitable bluffs. If you fold more than the MDF, your opponent makes money bluffing any two cards. The formula: MDF = pot ÷ (pot + bet). Against a half-pot bet, MDF = 100 ÷ 150 ≈ 67%; against a pot-sized bet, 50%; against a 2× pot overbet, 33%. The bigger the bet, the less you're required to defend — it's the exact complement of the price the bluffer pays (their bluff must succeed bet ÷ (pot + bet) of the time to break even). MDF is a theoretical compass, not an absolute law: it assumes an opponent capable of bluffing at the right frequency. Against most players, who under-bluff big river sizings, over-folding relative to MDF is a correct exploitative deviation. It remains the reference tool for auditing your own frequencies: if you fold 70% against half-pot c-bets, you're a target.
River, 12bb pot, your opponent bets 6bb (half pot). MDF = 12 ÷ 18 ≈ 67%: you must continue with about two-thirds of your range to stay unexploitable. Their bluff, meanwhile, needs to work 6 ÷ 18 = 33% of the time to break even. If you fold 50% of the time, every opponent bluff prints money.