title: Construct validation of the Amblyopia and Strabismus Questionnaire (A&SQ) by factor analysis.
abstract: BACKGROUND
  The Amblyopia and Strabismus Questionnaire (A&SQ) was previously developed to assess quality of life (QoL) in amblyopia and/or strabismus patients. Here, factor analysis with Varimax rotation was employed to confirm that the questions of the A&SQ correlated to dimensions of quality of life (QoL) in such patients.
  
  
  METHODS
  Responses on the A&SQ from three groups were analyzed: healthy adults (controls) (n = 53), amblyopia and/or strabismus patients (n = 72), and a historic cohort of amblyopes born between 1962-1972 and occluded between 1968-1974 (n = 173). The correlations among the responses to the 26 A&SQ items were factor-analysed by Principal Component Analysis (PCA). As the development of the A&SQ was intuitive-deductive, it was expected that the pattern of correlation could be explained by the five a priori hypothesized dimensions: fear of losing the better eye, distance estimation, visual disorientation, diplopia, and social contact and cosmetic problems. Distribution of questions along the factors derived by PCA was examined by orthogonal Varimax rotation.
  
  
  RESULTS
  Data from 296 respondents were analyzed. PCA provided that six factors (cutoff point eigenvalue >1.0) accumulatively explained 70.5% of the variance. All A&SQ dimensions but one matched with four factors found by Varimax rotation (factor loadings >0.50), while two factors pertained to the fifth dimension. The six factors explained 33.7% (social contact and cosmetic problems); 10.3% (near distance estimation); 8.7% (diplopia); 7.2% (visual disorientation); 6.3% (fear of losing the better eye); and 4.3% (far distance estimation), together 70.48% of the item variance.
  
  
  CONCLUSIONS
  The highly explained variance in the A&SQ scores by the factors found by the PCA confirmed the a priori hypothesized dimensions of this QoL instrument.
authors.affiliation: Department of Ophthalmology, Erasmus MC, University Medical Center Rotterdam, NL-3000 CA Rotterdam, The Netherlands.
authors.forename: Elizabeth S
authors.initials: ES
authors.lastname: van de Graaf
authors.forename: Joost
authors.initials: J
authors.lastname: Felius
authors.forename: Hanneke
authors.initials: H
authors.lastname: van Kempen-du Saar
authors.forename: Casper W N
authors.initials: CW
authors.lastname: Looman
authors.forename: Jan
authors.initials: J
authors.lastname: Passchier
authors.forename: Henk
authors.initials: H
authors.lastname: Kelderman
authors.forename: Huibert J
authors.initials: HJ
authors.lastname: Simonsz
mesh_terms: D000328:Adult; D000550:Amblyopia / Q000523:psychology* / Q000628:therapy; D005163:Factor Analysis, Statistical; D005260:Female; D006801:Humans; D008297:Male; D011788:Quality of Life; D012683:Sensory Deprivation; D013285:Strabismus / Q000523:psychology* / Q000628:therapy; D011795:Surveys and Questionnaires; D015348:Vision, Binocular; D014792:Visual Acuity