In regard to outfit, however, three items (2, 21 and 27) presented unexpected pattern of answers, indicating a discrepancy in the answers related to the sample's theta level in comparison to the item's level of difficulty (i.e., children with theta levels above or below these item's level of difficulty were not answering as expected).
First, the item analyses we conducted were based on the Classical Test Theory (CTT), which has several limitations in comparison to the Item Response Theory (IRT) due to its inter-dependence of item and participants.
Although multiple item reviews and a pilot trial had been conducted in advance, we cannot exclude that KFP and PST items were more difficult in comparison to the CKT items.
One relevant item of the oral symptoms domain is concerning the child's pain in the teeth or mouth, which showed a higher frequency in comparison to the other items in this domain.
The anchor point of the items have varied between studies [ 4, 11] and there is a difference in precision of the description of the clinical observations in comparison to the other items which are described more precisely.
An examination of the x-axis for expected total score shows all items were rated from a minimum total score of seven to the maximum total score for the Positive Symptoms subscale, and were rated as "Yes" for Criterion 4. From examination of Figure 2 in comparison to the ideal item presented in Figure 1, it can be observed that items P1, P3, P4 and P6 most closely resemble the ideal item.
We analyzed the PF, EF, SF CF, and global QOL domains in comparison to the corresponding SSQ items.
Good stability of content validity was observed even in the extreme case in which only 7 8 items were retained (note in that case the loss of variance explained in comparison to the initial 23-item model was only 1%).
The decision to exclude Guttman scaling was based on the considerable evidence stating the inferiority of these methods in comparison to the more advanced item response methods [ 33].
The purpose of this study was to evaluate the performance of the 11-item QuickDASH in comparison to the full-length 30-item DASH regarding different aspects of validity and reliability.
