The document discusses a Rasch analysis of the Dermatology Life Quality Questionnaire (DLQI), a 10-item patient-reported outcome measure used to assess quality of life for patients with various skin conditions. The analysis identified several issues with the DLQI, including misfitting items, disordered response thresholds, and differential item functioning. This suggests problems with the scale's development methodology and raises concerns about its use in clinical trials and treatment decisions. The document advocates for improved scale development using modern psychometric methods like Rasch analysis and qualitative research with patients.
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Rasch analysis of the Dermatology Life Quality Questionnaire (DLQI)
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
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
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)
Notes de l'éditeur
Aim: to discuss how problems identified during Rasch analysis can be used to illustrate scale development issues
One of the most popular HRQoL measures in Dermatology
NICE guidelines & Health Technology Assessment (HTA)
Developed in 1994 before the wide use of Rasch analysis in the area of health outcomes
Here in lies several of the problems.
Too much condensing of information. Too many ideas are squeezed into each item. Items should be simple, clear and contain only one idea. This is important from the perspective of Rasch analysis. Failure to follow this produces scales which have built in error.
No theoretical model
The content of the DLQI mainly covers issues related to functioning
There is one emotional related item
You can also see from this the items try to cover several areas in one item
One may argue that items should not be included if they are not relevant to some of the patients
A side issue is that individuals who answered not relevant were given the same score as those that answered ‘not at all’
Psoriasis sample from the Manchester Psoriasis Service at Hope Hospital
Atopic Dermatitis sample from the National Eczema society
Mean fit residual for item-person interaction was extreme indicating misfit to the Rasch model.
Shows the degree of divergence between the expected and actual data for each item. Transformed into a Z-score.
Unidimensionality t-test was fine – suggesting that there was not multiple factors within the scale – Everett Smith (2002)
Items 2, 5 and 7 all have some indication of problems
Item 2 – Why does this item not fit the Rasch model? – Is it because this is the only emotional item? It is possible that it just doesn’t fit with the other items
Item 5 – This is a very poorly worded item. The target audience would not use this kind of language. This may not be the reason for the item problem but it certainly doesn’t help. Items should be simple and in a language that is easy for respondents to relate to.
It is unclear exactly why this item showed DIF by working or studying.
This does, however, suggest that scores for participants with different kinds of skin conditions should not be combined
DLQI 4 – obvious gender bias with the item
If data were Rasch analysed during the development stage then some of these issues could have been addressed
Category probability curve
Several items with disordered response thresholds
Response option 2 ‘a lot’ is also poorly defined in all items
Response threshold issues were due to poor response option choices
‘Not relevant’ responders were scored the same as ‘not at all’ responders
Results could underestimate problems with scoring of the response options
Ambiguous nature of some of the items may have contributed to some of the problems
This another issue which could again have been highlighted at the development stage
DLQI scores cannot be compared across diseases without splitting of items
The DLQI lacks items covering mild levels of HRQoL in dermatological conditions
Problematic scaling issues are due to fundamental problems with the DLQI development
Based on these findings it has been shown how failure to follow principals of good questionnaire design can lead to inbuilt error into the scale
Simple and clear items, good measurement spread, items that are relevant, good response options, free from DIF