The probability We also assume that p(d) = 1 when d is an empty sequence of rules.
the probability p(w) of a string w is the sum of all complete derivations deriving that string from the start symbol: With this notation, consistency of a PCFG is de-1214 fined as the condition: a PCFG is consistent if the sum of probabilities of all complete derivations starting with S is 1.