I am refering to Introduction to Quantum Mechanics (2nd Edition) by David J. Griffiths, page 425 on Bell's Theorem. Griffiths used a parameter, called [tex]\rho(\lambda)[/tex] as the probability density for the hidden variable. What I don't understand is that the hidden variable was suppose to make the theory deterministic, or specifically to show that quantum mechanics as an indeterministic theory is incomplete. What is the reason that he can use probability to describe the hidden variable? Isn't this a contradiction?