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Rasch analysis of the Roland-Morris Disability Questionnaire

Rasch analysis of the Roland-Morris Disability Questionnaire. Megan Davidson, PhD School of Physiotherapy, La Trobe University, Melbourne. Functioning Disability Ability Health status Activity limitations Participation restrictions Quality of life Well being Level of assistance.

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Rasch analysis of the Roland-Morris Disability Questionnaire

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  1. Rasch analysis of the Roland-Morris Disability Questionnaire Megan Davidson, PhD School of Physiotherapy, La Trobe University, Melbourne

  2. Functioning Disability Ability Health status Activity limitations Participation restrictions Quality of life Well being Level of assistance Typically have n items summed to give a total score Higher score indicates more or less of the “thing” being measured Questionnaires

  3. The scores are ordinal • Rank order • Distance between ranks is unknown • Distance between adjacent scores are not equivalent units • Arguably, ordinal scores cannot be manipulated mathematically

  4. How far will the cars go? • 4 cars are filled with petrol to see how far they go on a full tank of fuel. • 3 observers are positioned along the route.

  5. Green: Score 3 Blue: Score 2 Red: Score 2 Yellow: Score 1

  6. Rasch analysis • Modern Test Theory (Item response) • Models the probability that a person of θ ability will be able to do activity of δ difficulty • Locates item difficulty and person ability on an interval-level logit scale • Logit = log-odds unit • probability a person can perform the task divided by the probability they cannot

  7. Advantage of Rasch modelling • Measure what is measurable, and make measurable what is not so. (Galileo Galilei) • “Rasch… provides an operational criterion for fundamental measurement of the kind found in the physical sciences” David Andrich

  8. Easy activities Hard activities Get out of bed Housework Gardening Sport Least able person Most able person

  9. Roland-Morris Questionnaire (RDQ) • A low-back specific disability questionnaire • 24-items from the Sickness Impact Profile • Patient self-completed • Tick those that apply “today” • Number of items selected = score • Possible score 0-24 • Higher score indicates greater disability

  10. Roland-Morris Disability Questionnaire • I stay at home most of the time because of my back • I change position frequently to try and get my back comfortable • I walk more slowly than usual because of my back • Because of my back, I am not doing any of the jobs that I usually do around the house

  11. RDQ content • housework • self-care • walking • sleeping • sitting • irritability • appetite • pain

  12. Short-form versions RDQ • 18-item version • Stratford & Binkley 1997 • 18-item version • Williams & Myers 2001

  13. Classical (Traditional) Test Theory • Reject/retain items on some basis • Very low or high response frequency • Very low or high item-item correlations • Low or high corrected item-total correlations • Cronbach’s alpha in range considered desirable

  14. Stratford & Binkley Response frequency <20% or >90% Item-item correlations > 0.75 Item-total correlations < .40 Increased Cronbach’s alpha Williams & Myer Response frequency <20 or >80% Item-item correlations > 0.75 Item-total correlations < .20 Cronbach’s alpha >.80 18-item versions

  15. Stratford & Binkley 2 change position 15 appetite not good 17 walk short distance 19 dress with help 20 sit most of day 24 stay in bed Williams & Myers 2 change position 15 appetite not good 19 dress with help 20 sit most of day 22 more irritable 24 stay in bed Items removed in 18-item versions

  16. Aim: • To examine fit to a Rasch model of the 24-item and two 18-item versions of the RDQ • To explore whether decisions to reject items on the basis of Rasch analysis would differ from that made by the developers of two 18-item versions of the RDQ.

  17. Is RDQ unidimensional? • Items drawn from several SIP domains • Williams & Myers 2001 • Many low item-item and some low item-total correlations • 4 factors explaining 55% of total variance

  18. Method • Data for 140 people from a previous study • Battery of questionnaires including RDQ • Participants were seeking physiotherapy treatment for a low back problem • aged 18 years or older • read and write English. • Recruited from public hospitals, community health centres and private practices. • RUMM2020 Rasch analysis software

  19. Results (n = 140) • Mean age 51 years (sd 17, range 18-89) • 66% female • 41% employed • 43% pain < 6 weeks • 34% pain > 6 months • 70% pain that referred into the buttock or leg. • RDQ score 9 (sd 5.6), median 8 (IQR 5-14).

  20. COMPARISON OF FIT TO THE RASCH MODEL Poor fit if item-trait p <.05, item residual > ±2, 2 p < .01, F p < .01 PSI = Person Separation Index

  21. Differential Item Functioning (DIF) • An item may attract systematically different responses on the basis of some characteristic other than item difficulty • Age • Gender • DIF by age and gender in all versions • Item 5 Because of my back, I use a handrail to get upstairs

  22. Which items would Rasch reject? • 17 walk short distances • 9 dress slowly • Negative residuals indicate redundancy • 5 use handrail upstairs • DIF by age/gender • 16 trouble putting on socks • DIF by age, another item at same location on logit scale

  23. 20-items fit the Rasch model • Item-Trait interaction • Total item chi square > .05: p = .424 • Item Fit • Item residuals all < ±2.0 • Item Chi Square and F-stat all p >.01 • No DIF by age and gender • PSI = 0.83

  24. Items removed • Stratford & Binkley version: 2,15,17,19,20,24 • Williams & Myers version: 2,15,19,20,22,24 • Rasch 21-item version: 5,9,16,17

  25. 24-item RDQ: Targetting 21 avoid heavy jobs -2.35 19 dress with help 4.12 Increasing item difficulty Increasing person ability Decreasing item difficulty Decreasing person ability

  26. 24-item RDQ: Targetting gap gap gap cluster Decreasing item difficulty Decreasing person ability Increasing item difficulty Increasing person ability

  27. 24-item RDQ 19 Dress with help 2 Change position frequently 17 Walk short distances 24 Stay in bed 15 Appetite not good 20 Sit most of the day 18-item RDQ Stratford

  28. 24-item RDQ 18-item RDQ Williams

  29. 24-item RDQ 20-item Rasch selection

  30. Raw Scores Vs Rasch Measure Change of 5 points from 10 to 5 = 1.16 logits Change of 5 points from 5 to 0 = 2.58 logits

  31. Conclusions • Traditional and Modern Test Theory approaches reject different items • Rejecting items of very low/high frequency results in truncated scale • RDQ can be made to fit Rasch model, but targetting is poor • Gaps in item difficulty locations • No items of sufficient difficulty for high ability persons

  32. Rasch Group Meeting Swinburne Hawthorn Monday 5th Dec, 4.00pm Room AR103 Graduate School of Research (next door to Haddon’s coffee shop in campus centre) Megan Davidson m.davidson@latrobe.edu.au Julie Pallant JPallant@groupwise.swin.edu.au

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