Analysis of internal construct validity of the SRS-24 questionnaire

The SRS-24 questionnaire was originally validated using methods of classical test theory, but internal construct validity has never been shown. Internal construct validity, i.e. unidimensionality and linearity, is a fundamental arithmetic requirement and needs to be shown for a scale for summating any set of Likert-type items. Here, internal construct validity of the SRS-24 questionnaire in adolescent idiopathic scoliosis (AIS) patients is analyzed.

232 SRS-24 questionnaires distributed to 116 patients with AIS pre-operatively and at postoperative follow-up were analyzed. 103 patients were females; the average age was 16.5 ± 7.1 years. The questionnaires were subjected to Rasch analysis using the RUMM2020 software package.

All seven domains of the SRS-24 showed misfit to the Rasch model, and three of seven were unidimensional. Unidimensionality and linearity could only be achieved for an aggregate score by separating pre- and postoperative items and omitting items which caused model misfit. Reducing the questionnaire to six pre-operative items (p = 0.098; 2.25% t tests) and five postoperative items (p = 0.267; 3.70% t tests) yields model fit and unidimensionality for both summated scores. The person-separation indices (PSI) were 0.67 and 0.69, respectively, for the pre- and postoperative patients.

The SRS-24 score is a non-linear and multidimensional construct. Adding the items into a single value is therefore not supported and invalid in principle. Making profound changes to the questionnaire yields a score which fulfills the properties of internal construct validity and supports its use a change score for outcome measurement.