Rasch analysis of three versions of the Oswestry Disability Questionnaire for Low Back Pain
From: Manual Therapy. Volume 13, Issue 3, Pages 222-231 (June 2008)
The Oswestry Disability Questionnaire is one of the oldest self-report questionnaires for measuring functional outcomes in patients with low back pain and remains widely used. The Oswestry Low Back Pain Disability Questionnaire was developed as a clinical assessment tool that would provide an estimate of disability expressed as a percentage score. Ten sections or items assess pain, personal care, lifting, walking, sitting, standing, sleeping, sex life, social life and travelling. The developers provided little detail on how the items were selected, saying only that the activities chosen were those most relevant to people with low back pain.
Each item of the Oswestry Low Back Pain Disability Questionnaire has 6 response choices arranged in order of difficulty and the respondent is asked to select the response that most closely describes you today. For example, the Sitting section responses are I can sit in any chair as long as I like, I can only sit in my favourite chair as long as I like, Pain prevents me sitting more than 1hr, Pain prevents me from sitting more than 30min, Pain prevents me from sitting more than 10min and Pain prevents me from sitting at all. A score of 0 is awarded if the first response option is selected, through to 5 for the last option. A total score is calculated by summing the individual items scores, dividing by the total possible score (adjusted if any items are missed) and multiplied by 100. The possible score range is 0–100 and a higher score indicates greater disability. The Oswestry Low Back Pain Disability Questionnaire is therefore an atypical questionnaire because there is no consistent rating scale used across all items: instead, each step of each item has its own definition.
The Oswestry Low Back Pain Disability Questionnaire was modified by Baker et al. who removed references to medication from the Pain and Sleeping items, thereby improving the relevance of these items to people not taking medication. Davidson and Keating further modified this version by replacing miles with kilometres in the Walking section. A modified version sometimes called the Chiropractic version replaced Sex Life with a new item called Changing Degree of Pain. This version has been criticised for including a transitional rating, which is conceptually different from the other items that ask about pain intensity and activity limitations. More recently, Fritz and Irrgang reported a version that replaced Sex Life with a new item called Employment/ Homemaking. This modification added an aspect of activity/participation that is otherwise absent from the Oswestry Low Back Pain Disability Questionnaire. The developers recommend Version 2.0 of the Oswestry, which instructs patients to answer the questions in relation to how their back problem is affecting them today, rather than the original instructions, which do not specify a time-frame. Selection of any particular version of the Oswestry Low Back Pain Disability Questionnaire is at present based solely on preference for content and no studies have directly compared different versions.
The aim of this study was to explore the construct validity of three versions of the Oswestry Low Back Pain Disability Questionnaire using Rasch analysis. This study directly compared three versions of the Oswestry Disability Questionnaire, which was achieved by administration of a test version containing the 10-item Oswestry plus two additional items contained in two modified Oswestry versions. Including all 12 items in the test version allowed all three versions to be extracted.
The version that replaces Sex Life with Work/Housework had the best overall fit to the model and the standard Oswestry showed adequate fit to the Rasch model, while the version that replaces Sex Life with Changing Degree of Pain did not. The results confirm that the item Changing Degree of Pain does not belong with the other items and is measuring a different underlying construct to the other items.
Testing that sub-sets of items provide an equivalent estimate of person ability to the entire set of items provides a robust indication of whether departures from unidimensionality significantly distort estimates of person location. Despite the presence of one misfitting item and overall poor fit to the model, Version 3 showed that the deviation from unidimensionality did not result in significantly different person location estimates calculated for two anchored subsets of items. It remains to be demonstrated how robust the estimates of person location are and the extent of departure from unidimensionality that can be tolerated before significant deviations occur.
Two previous studies that have used Rasch analysis to examine the Oswestry Low Back Pain Disability Questionnaire reported that the Pain item did not fit the model. That the current study did not find this item misfitting may reflect the different wording of the item in the versions administered. The version administered in the current study asks only about pain intensity, while the two previous studies have administered a version that relates pain to analgaesic medication. If the content of the Oswestry Low Back Pain Disability Questionnaire is mapped to the WHO International Classification of Functioning the pain item is a measure of impairment while the other items reflect activity limitations. However, these items all relate activity limitation to pain, and the pain item has been shown to have a linear relationship with the other items.
The existence of disordered thresholds for Personal Care, Standing, Sex Life and Social Life is evidence that, at least for these items, the response options do not perform as intended. White and Velozo and Page et al. both proposed modified versions of the Oswestry in which the Pain item is deleted and response levels 2 and 3, and 4 and 5 for all items are combined, reducing the number of response options from 6 to 4. Neither study reports which individual items had disordered thresholds. Citation tracking has failed to find any subsequent studies that have administered or further tested either of these versions. Due to the low frequency of responses to response options 4 and 5 in an ambulatory population there is some merit in suggesting a reduction in response options at the upper end of the scale.
Item 4 Walking displayed differential item functioning by age in all three versions. On this item, persons in the 65-plus age group, at the same level of ability as the younger groups, had higher (worse) scores than expected. This indicates that something other than the difficulty of walking as an activity is influencing older persons’ responses to this item. Fear of falling and various sociodemographic variables have been reported to be associated with reduced mobility in elderly persons. Neither of the previous Rasch studies reported if they examined differential item functioning.
The Oswestry item thresholds for the persons in this sample are a reasonable match in that there are thresholds for all persons except for a small number of persons of very high ability. This reflects the fact that total Oswestry scores in an ambulatory population are often skewed toward the lower (better functioning) end of the scale, with few persons scoring in the top 1/5th of the available total score range. This is because the highest responses options of some items are rarely or never selected. Some gaps are evident in the item difficulty threshold placement on the logit scale on the far right (lower functioning) end of the scale. Ideally, item thresholds should be evenly spread along the logit scale.
Although two versions showed adequate overall fit to the Rasch model, the problems of disordered thresholds for some items, differential item functioning for the Walking item, and gaps in targeting are typical of the limitations of ordinal scales designed using classical test theory and which are only revealed using Rasch anlysis.
A limitation of the study is that the number of eligible patients who were not invited or who refused to participate in the study is unknown. It is also not known the extent to which the sample is representative of ambulatory patients seeking physiotherapy treatment for low back pain, as there is no data available to describe this population. However, participants were recruited from a number of private and public agencies in both metropolitan and rural settings and this would maximise the likelihood that the sample is representative. As the data were collected from ambulatory patients with low back pain no generalisations can be made to non-ambulatory or admitted patients.
The standard version of the Oswestry and the version that replaces Sex Life with Work/Housework both form unidimensional scales in which all items are measuring a single underlying variable. The item Changing Degree of Pain that replaces Sex Life in the Hudson–Cook version does not measure the same underlying construct as the other items. These findings suggest that either of the first two of the three versions of this widely used low back pain outcome measure should be selected over the third. Users should also be aware that for some items the rating scale steps do not perform as intended.
Item 1: pain intensity
– I have no pain at the moment
– The pain is very mild at the moment
– The pain is moderate at the moment
– The pain is fairly severe at the moment
– The pain is very severe at the moment
– The pain is the worst imaginable at the moment
Item 2: personal care (washing, dressing, etc.)
– I can look after myself normally without causing extra pain
– I can look after myself normally but it is very painful
– It is painful to look after myself and I am slow and careful
– I need some help but manage most of my personal care
– I need help every day in most aspects of self-care
– I do not get dressed, wash with difficulty and stay in bed
Item 3: lifting
– I can lift heavy weights without extra pain
– I can lift heavy weights but it gives extra pain
– Pain prevents me lifting heavy weights off the floor but I can manage if they are conveniently positioned, e.g. on a table
– Pain prevents me lifting heavy weights but I can manage light to medium weights if they are conveniently positioned
– I can only lift very light weights
– I cannot lift or carry anything
Item 4: walking
– Pain does not prevent me walking any distance
– Pain prevents me from walking more than 2km
– Pain prevents me from walking more than 1km
– Pain prevents me from walking more than 500m
– I can only walk using a stick, crutches or other support
– I am unable to walk at all
Item 5: sitting
– I can sit in any chair as long as I like
– I can only sit in my favourite chair as long as I like
– Pain prevents me sitting for more than 1h
– Pain prevents me from sitting for more than 30min
– Pain prevents me from sitting more than 10min
– Pain prevents me from sitting at all
Item 6: standing
– I can stand as long as I want without extra pain
– I can stand as long as I want but it gives me extra pain
– Pain prevents me from standing for more than 1h
– Pain prevents me from standing for more than 30min
– Pain prevents me from standing for more than 10min
– Pain prevents me from standing at all
Item 7: sleeping
– My sleep is never disturbed by pain
– My sleep is occasionally disturbed by pain
– Because of pain I have less than 6h sleep
– Because of pain I have less than 4h sleep
– Because of pain I have less than 2h sleep
– Pain prevents me from sleeping at all
Item 8: sex life (if applicable)
– My sex life is normal and causes no extra pain
– My sex life is normal but causes some extra pain
– My sex life is nearly normal but is very painful
– My sex life is severely restricted by pain
– My sex life is nearly absent because of pain
– Pain prevents any sex life at all
Item 9: social life
– My social life is normal and gives me no extra pain
– My social life is normal but increases the degree of pain
– Pain has no significant effect on my social life apart from limiting my more energetic interests e.g. sport, etc.
– Pain has restricted my social life and I do not go out as often
– Pain has restricted my social life to my home
– I have no social life because of pain
Item 10: travelling
– I can travel anywhere without pain
– I can travel anywhere but it gives extra pain
– Pain is bad but I manage journeys over 2h
– Pain restricts me to journeys of less than 1h
– Pain restricts me to short necessary journeys under 30min
– Pain prevents me from travelling except to receive treatment
Item 11: work/housework
– My normal work/housework does not cause pain
– My normal work/housework increase my pain, but I can still perform all that is required of me
– I can perform most of my work/housework, but pain prevents me from performing more physically demanding activities (e.g., lifting, vacuuming)
– Pain prevents me from doing anything but light work/housework
– Pain prevents me from doing even light work/housework
– Pain prevents me from performing any work/housework
Item 12: changing degree of pain
– My pain is rapidly getting better
– My pain fluctuates but overall is definitely getting better
– My pain seems to be getting better but improvement is slow at present
– My pain is neither getting better or worse
– My pain is gradually worsening
– My pain is rapidly worsening
