Fig. 2

Person-item map for the six items in VQ6 showing the ability of the patients to the left and the difficulty of the items (VQ1-VQ6) to the right of a linear scale (from −6 to +6 logits). The measure (logit) shows the logarithm of the odds ratio for being able to perform an item activity successfully. The distance between a respondent and an item shows the probability of the respondents to perform successfully the item activity. Preferable, person and item should center on the same mean value. M, S and T represent mean, 1SD and 2SD, respectively. Each # is 2 persons. The instrument is well targeted because the item means and person means were separated by only 0.48 logits