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.
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 psoriasispatients. METHODS: PFI data were collected from 150 accompanying family members of psoriasispatients 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 psoriasispatients. 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.