Hypothetical item response curves for SUD diagnostic criteria. The item depicted by curve 1 is a criterion likely to be met by all individuals, except those with very low risk of SUD. It discriminates well between those who meet it and those who do not because the curve is steep, and this is also true curve 2, which has the same steepness but distinguishes between individuals 1 s.d. higher on the liability distribution. Curve 3 is the flattest and is consequently the least discriminating because the probability of showing this symptom changes more slowly as the risk of SUD increases. Curve 4 discriminates perfectly, but it is only useful for measuring individuals close to the 1.5 s.d. point on the distribution.
