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.
Objectives
• To discuss good scale development
methodology in the context of Rasch analysis
• Example - Dermatology Life Quality Index
(DLQI)

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.
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.
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.
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.
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.
Study aims
• Rasch analysis of DLQI data from two patient
groups:
- atopic dermatitis
- psoriasis
• Relate results to development methods used for
DLQI

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.
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.
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.
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.
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.
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.
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.
Example of DIF by disease
Prevents working or studying
Person locations (logits)
Expected value
Psoriasis
Atopic Dermatitis

17.
Example of DIF by gender
Influences the clothes you can wear
Expected value
Person locations (logits)
Females
Males

18.
Response thresholds

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.
Item Map
Clustering of
items
Poor
coverage of
persons

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.
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.
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.
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)