title: PFI-14©: A Rasch Analysis Refinement of the Psoriasis Family Index.
abstract: OBJECTIVE
  We aimed to assess the psychometric properties of the Psoriasis Family Index (PFI), a disease-specific instrument, to measure the secondary impact of psoriasis on the quality of life (QoL) of family members and partners of psoriasis patients.
  
  
  METHODS
  PFI data were collected from 150 accompanying family members of psoriasis patients attending the dermatology department of a secondary referral hospital. Rasch analysis was used to examine various psychometric properties of the PFI, including dimensionality, response category functioning, fit statistics, scale reliability and validity, item targeting and differential item functioning (DIF).
  
  
  RESULTS
  The 15-item PFI with 4 response options showed good overall fit to the Rasch model. The response category almost entirely followed the Linacre criteria, demonstrating its optimal functionality. All items except one had good fit to the Rasch model. The mean fit residual for the items was -0.085 ± 1.184 (range -1.54 to 2.7). Strict unidimensionality was achieved following removal of the misfitting item 5. The revised 14-item PFI maintained strong reliability (person separation index = 0.87). The scale showed minor mistargeting (mean difference between person and item location = 0.95 logits). None of the items displayed DIF across gender or relationship type.
  
  
  CONCLUSIONS
  The PFI is the first disease-specific measure to quantify the QoL of family members of psoriasis patients. Following the application of Rasch analysis we now recommend the use of a 14-item version of the PFI: question No. 5 from the 15-item version has been deleted. This study demonstrates the promising psychometric performance of the PFI and confirms its potential as a useful outcome measure in future clinical research.
authors: Department of Dermatology, Derriford Hospital, Plymouth, UK.
authors: Mohammad K A
authors: MK
authors: Basra
authors: Anne-Marie
authors: AM
authors: Zammit
authors: Paul
authors: P
authors: Kamudoni
authors: Ahmed M
authors: AM
authors: Eghlileb
authors: Andrew Y
authors: AY
authors: Finlay
authors: M Sam
authors: MS
authors: Salek
mesh_terms: D005260:Female; D006801:Humans; D008297:Male; D015233:Models, Statistical; D010290:Parents / Q000523:psychology; D011565:Psoriasis / Q000523:psychology*; D011594:Psychometrics; D011788:Quality of Life / Q000523:psychology*; D015203:Reproducibility of Results; D018454:Spouses / Q000523:psychology; D011795:Surveys and Questionnaires