European Spine Journal

, Volume 21, Issue 8, pp 1590–1595 | Cite as

Analysis of internal construct validity of the SRS-24 questionnaire

  • Dominique A. Rothenfluh
  • Georg Neubauer
  • Juergen Klasen
  • Kan Min
Original Article

Abstract

Purpose

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.

Methods

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.

Results

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.

Conclusions

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.

Keywords

SRS-24 Rasch analysis Internal construct validity 

Notes

Conflict of interest

The authors declare no competing interest.

Supplementary material

586_2012_2169_MOESM1_ESM.docx (71 kb)
Supplementary material 1 (DOCX 71 kb)

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Copyright information

© Springer-Verlag 2012

Authors and Affiliations

  • Dominique A. Rothenfluh
    • 1
  • Georg Neubauer
    • 1
  • Juergen Klasen
    • 1
  • Kan Min
    • 1
  1. 1.Department of OrthopaedicsBalgrist Clinic, University of ZurichZurichSwitzerland

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