title: Rasch analysis and impact factor methods both yield valid and comparable measures of health status in interstitial lung disease.
abstract: OBJECTIVE
  Rasch analysis has largely replaced impact factor methodology for developing health status measures. The aim of this study was to develop a health status questionnaire for patients with interstitial lung disease (ILD) using impact factor methodology and to compare its validity with that of another version developed using Rasch analysis.
  
  
  METHODS
  A preliminary 71-item questionnaire was developed and evaluated in 173 patients with ILD. Items were reduced by the impact factor method (King's Brief ILD questionnaire, KBILD-I) and Rasch analysis (KBILD-R). Both questionnaires were validated by assessing their relationship with forced vital capacity (FVC) and St Georges Respiratory Questionnaire (SGRQ) and by evaluating internal reliability, repeatability, and longitudinal responsiveness.
  
  
  RESULTS
  The KBILD-R and KBILD-I comprised 15 items each. The content of eight items differed between the KBILD-R and KBILD-I. Internal and test-retest reliability was good for total scores of both questionnaires. There was a good relationship with SGRQ and moderate relationship with FVC for both questionnaires. Effect sizes were comparable. Both questionnaires discriminated patients with differing disease severity.
  
  
  CONCLUSIONS
  Despite considerable differences in the content of retained items, both KBILD-R and KBILD-I questionnaires demonstrated acceptable measurement properties and performed comparably in a clinical setting.
authors.affiliation: Division of Asthma, Allergy and Lung Biology, King's College London, Denmark Hill, London SE5 9RS, UK.
authors.forename: Amit S
authors.initials: AS
authors.lastname: Patel
authors.affiliation: Person Centred Research Centre, Health and Rehabilitation Research Institute, AUT University, North Shore campus, 90 Akoranga Drive, Northcote, Auckland 0627, New Zealand.
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Siegert
authors.affiliation: Department of Palliative Care, Policy and Rehabilitation, Cicely Saunders Institute, King's College London, Bessemer Road, London SE5 9PJ, UK.
authors.forename: Sabrina
authors.initials: S
authors.lastname: Bajwah
authors.affiliation: Division of Asthma, Allergy and Lung Biology, King's College London, Denmark Hill, London SE5 9RS, UK.
authors.forename: Kate
authors.initials: K
authors.lastname: Brignall
authors.affiliation: Department of Respiratory Medicine, NUTRIM School for Nutrition, Toxicology and Metabolism, Maastricht University Medical Center, Minderbroedersberg 4-6, 6211 LK Maastricht, The Netherlands.
authors.forename: Harry R
authors.initials: HR
authors.lastname: Gosker
authors.affiliation: Division of Asthma, Allergy and Lung Biology, King's College London, Denmark Hill, London SE5 9RS, UK.
authors.forename: John
authors.initials: J
authors.lastname: Moxham
authors.affiliation: Interstitial Lung Disease Unit, Royal Brompton Hospital, Sydney Street, London SW3 6NP, UK.
authors.forename: Toby M
authors.initials: TM
authors.lastname: Maher
authors.affiliation: Interstitial Lung Disease Unit, Royal Brompton Hospital, Sydney Street, London SW3 6NP, UK.
authors.forename: Elisabetta A
authors.initials: EA
authors.lastname: Renzoni
authors.affiliation: Interstitial Lung Disease Unit, Royal Brompton Hospital, Sydney Street, London SW3 6NP, UK.
authors.forename: Athol U
authors.initials: AU
authors.lastname: Wells
authors.affiliation: Department of Palliative Care, Policy and Rehabilitation, Cicely Saunders Institute, King's College London, Bessemer Road, London SE5 9PJ, UK.
authors.forename: Irene J
authors.initials: IJ
authors.lastname: Higginson
authors.affiliation: Division of Asthma, Allergy and Lung Biology, King's College London, Denmark Hill, London SE5 9RS, UK. Electronic address: Surinder.birring@nhs.net.
authors.forename: Surinder S
authors.initials: SS
authors.lastname: Birring
mesh_terms: D005260:Female; D006304:Health Status; D006801:Humans; D017563:Lung Diseases, Interstitial / Q000503:physiopathology*; D008297:Male; D008875:Middle Aged; D011237:Predictive Value of Tests; D011446:Prospective Studies; D011788:Quality of Life; D015203:Reproducibility of Results; D012129:Respiratory Function Tests; D012720:Severity of Illness Index; D011795:Surveys and Questionnaires
keywords: Health status; Impact factor; Interstitial lung disease; KBILD; King's Brief Interstitial Lung Disease questionnaire; Quality of life; Rasch analysis