Usual system: take as scale value the normal deviate corresponding to the area p″ = (p − p0)/(1 − p0), where p is the proportion of correct answers and p0 is the chance proportion. If p ≦ p0, the item difficulty is unscalable. Proposed system: take as scale value the normal deviate corresponding to the area p′ = px, with x so chosen that p′ = .5 when p is half-way between p0 and 1. Table gives D (=500 + 100z) for p = .00(.01)1.00 and for a (= number of alternatives) 2,3,4,5,6,7, ∞.