In the above equation, θ represents ability and bi represents the value of the b-parameter for item i. The value of bi typically ranges between -2 and +2, but can take on more extreme values. Under the 1PL model, the value of bi represents the level of ability (θ) required for a 50% chance of correct response (i.e., .5 probability of correct response). Thus, if bi = 0, then the probability of correct response will equal .5 at an ability level of θ = 0.