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Rasch analysis of the Dermatology Life Quality Questionnaire (DLQI)

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The purpose of this presentation is to discuss good scale development methodology in the context of Rasch analysis.

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Rasch analysis of the Dermatology Life Quality Questionnaire (DLQI)

  1. 1. Rasch analysis of the Dermatology Life Quality Questionnaire (DLQI) James Twiss and Stephen McKenna Galen Research Ltd, Manchester, UK Email: jtwiss@galen-research.com
  2. 2. Objectives • To discuss good scale development methodology in the context of Rasch analysis • Example - Dermatology Life Quality Index (DLQI)
  3. 3. The DLQI • 10-item generic dermatology Patient Reported Outcome (PRO) measure • Scored 0-30. High scores=worse • Used with 30 different skin conditions • Available in 55 languages • Used for treatment selection in the UK
  4. 4. DLQI development • Uses patient reports of problems • 49 identified ‘aspects’ condensed into 10 items • Designed to fit on one side of A4 • Items phrased to include additional aspects
  5. 5. The DLQI - content Sample DLQI items: Over the last week… how embarrassed or self conscious have you been because of your skin? how much has your skin interfered with you going shopping or looking after your home or garden? how much has your skin made it difficult for you to do any sport? how much of a problem has the treatment for your skin been, for example by making your home messy, or by taking up time?
  6. 6. The DLQI – response format • Four point response format – Very much – A lot – A little – Not at all • 8 items also have a ‘Not relevant’ option
  7. 7. Psychometric properties • Classical psychometric properties adequate (Basra et al, 2008) • One previous Rasch analysis compared 6 language versions (Nijsten and Meads, 2007) – Overall misfit to the model – Misfit in 3 individual countries – DIF by country for every item
  8. 8. Study aims • Rasch analysis of DLQI data from two patient groups: - atopic dermatitis - psoriasis • Relate results to development methods used for DLQI
  9. 9. Analyses Rasch analysis employed RUMM2020 • Overall fit to model • Individual Item fit • Response threshold order • DIF by age and gender • DIF by disease (AD vs psoriasis) • Item-trait coverage
  10. 10. Sample Psoriasis (n = 146) Atopic Dermatitis (n = 146) Gender (%) Male 73 (50) 73 (50) Female 73 (50) 73 (50) Age (Years) Mean (SD) 44.4 (14.7) 45.5 (16.6) Range 66 (17-83) 62 (20-82) Duration (years) Mean (SD) 20.9 (13.5) 28.2 (17.5) Range 67 (2-69) 76 (0-76)
  11. 11. DLQI scores Psoriasis Atopic dermatitis Mean (SD) 8.8 (6.7) 6.1 (4.6) Median (IQR) 7 (3.0-12.3) 5 (3-8) Range 29 (0-29) 26 (0-26)
  12. 12. Overall fit to the Rasch model Initial fit statistics (partial credit model) Item-trait interaction PSI Items Persons Mean SD Mean SD 0.85 0.98 -0.30 0.81 0.01 -0.81
  13. 13. Item fit Item description Location Fit residual Chi2 p value 1. itchy, sore, painful or stinging -1.61 -0.03 3.4 0.50 2. embarrassment/self consciousness -0.52 -1.97 15.0 3. interferes with shopping/looking after home/garden 0.005* 0.99 -1.18 3.8 0.44 4. influences choice of clothes -0.54 -0.14 4.4 0.35 5. affects social/leisure activities 0.32 12.7 6. affect ability to do sport -0.02 0.29 3.0 0.56 7. prevents working/studying 0.25 0.23 12.6 8. creates problems with partner/close friends/relatives -2.69 0.01 0.01 0.59 -0.89 2.1 0.72 9. causes sexual difficulties 0.89 -1.23 2.2 0.70 10. problems with treatment -0.36 -0.47 3.5 0.48
  14. 14. Problems with items • 2 - Over the last week, how embarrassed or self conscious have you been because of your skin? • 5 - Over the past week, how much has your skin affected any social or leisure activities? • 7 - Over the past week, how much has your skin prevented you working / studying?
  15. 15. DIF analysis Item description Uniform DIF Non-uniform DIF itchy, sore, painful or stinging Disease embarrassment/self consciousness Age group/Gender interferes with shopping/looking after home/garden Disease Disease influences choice of clothes Gender*/ Disease affects social/leisure activities Disease affect ability to do sport Gender Gender prevents working/studying Gender/Disease* creates problems with partner/close friends/relatives causes sexual difficulties problems with treatment Age group*
  16. 16. Example of DIF by disease Prevents working or studying Person locations (logits) Expected value Psoriasis Atopic Dermatitis
  17. 17. Example of DIF by gender Influences the clothes you can wear Expected value Person locations (logits) Females Males
  18. 18. Response thresholds
  19. 19. Response options • Four point response format  Very much  A lot  A little  Not at all Source of problem for items 4, 7 and 8 Source of problem for items 6 and 9
  20. 20. Item Map Clustering of items Poor coverage of persons
  21. 21. Conclusions • DLQI – Pre-Rasch PRO • Rasch analysis highlighted several fundamental problems with DLQI • Problems probably result from inadequate scale development methodology • Concern for: - clinical trials - treatment decisions
  22. 22. Good PRO Design Item Reduction • Based on sound theoretical model • Cognitive debriefing interviews • Patient survey & application of Rasch analysis to data Draft 2 Draft 3 Scale Evaluation • Scaling properties • Classical psychometrics Draft 4 Item Generation • Patient Interviews • Qualitative analysis Draft 1
  23. 23. References Finlay AY, Khan GK (1994). Dermatology Life Quality Index (DLQI)-a simple practical measure for routine clinical use. Clin Exp Dermatol 19: 210-6. Basra MKA, French R, Gatt RM, et al(2008). The dermatology Life Quality Index 1994-2007: a comprehensive review of validation and clinical results. Br J Dermatol 159: 997-1035 Nijsten T, Meads DM, de Korte J et al (2007). Cross-Cultural Inequivalence of Dermatology-Specific Health related Quality of Life Instruments in Psoriasis Patients. Journal of Investigative Dermatology 127: 2315-2322 Nijsten T, Meads DM, McKenna SP (2006). Dimensionality of the dermatology life quality index (DLQI): a commentary. Acta Derm Venereol 86:284-5; author reply 285-6.
  24. 24. Overall fit to the model Item-Trait interaction PSI Unidimensionality (CI) Items Persons Mean SD Mean SD 0.01 0.85 -0.81 0.98 -0.30 0.81 0.03 (0.01 – 0.06)

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