From page 478 of CFA book? What is the market timing score of someone who flips a fair coin to predict the market? I inituivitly think the answer is zero, but I am not sure why? Can someone explain

sad but true the number is as good as most prof market timers whatever that number is i refuase to have to memorize this

P1+P2-1 P1 the probability of 1 face : 50% P2 the probability of the other : 50% P1+P2-1 : 0%