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. Author manuscript; available in PMC: 2017 Jan 1.
Published in final edited form as: Qual Life Res. 2015 Jun 29;25(1):13–23. doi: 10.1007/s11136-015-1058-8

Figure 2.

Figure 2

Upper panel: A hypothetical plot of the probability a pair of questionnaire responses is judged “different” as a logistic regression function of the scale score difference associated with the pair, with a dashed line indicating the MID at P = 0.5. Lower panel: The x-axis is the latent variable (propensity to respond “different”); the y-axis is the probability of responding “different.” The curves are IRT trace lines for six stimulus pairs, associated with scale score differences between 1.1 and 5.7 points. The normal population distribution is shown as a dotted line; the vertical dashed line is at the mean of the reference population, and an inferred trace line is drawn as a thicker line with its location (b) parameter equal to 0.0. The scale score difference associated with that thicker trace line is the estimate of the MID.