Analysis of internal construct validity of the SRS-24 questionnaire
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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.
KeywordsSRS-24 Rasch analysis Internal construct validity
Conflict of interest
The authors declare no competing interest.
- 3.Haher TR, Gorup JM, Shin TM, Homel P, Merola AA, Grogan DP, Pugh L, Lowe TG, Murray M (1999) Results of the scoliosis research society instrument for evaluation of surgical outcome in adolescent idiopathic scoliosis: a multicenter study of 244 patients. Spine (Phila Pa 1976) 24(14):1435–1440CrossRefGoogle Scholar
- 5.Merola AA, Haher TR, Brkaric M, Panagopoulos G, Mathur S, Kohani O, Lowe TG, Lenke LG, Wenger DR, Newton PO, Clements DH 3rd, Betz RR (2002) A multicenter study of the outcomes of the surgical treatment of adolescent idiopathic scoliosis using the scoliosis research society (srs) outcome instrument. Spine (Phila Pa 1976) 27(18):2046–2051CrossRefGoogle Scholar
- 7.Rasch G (1960) Probabilistic models for some intelligence and attainment tests. University of Chicago Press, ChicagoGoogle Scholar
- 9.Streiner D, Norman G (1989) Health measurement scales. Oxford University Press, OxfordGoogle Scholar