What is the HOOS-PS?
The HOOS-PS is a 5-item measure of physical functional derived from the items of
the Function, daily living and Function, sports and recreational activity subscales of
the HOOS. As with the HOOS it is intended to elicit people’s opinions about the difficulties
they experience with activity due to problems with their hip.
Given the overall objective, it was critical that each component be parsimonious and cross-culturally valid to enable use of the measure in multi-centre, cross-country studies and clinical settings. As a result, the decision was made to attempt to shorten the HOOS and to combine the items from the Function, daily living and Function, sports and recreational activity subscales. We chose to use an item response theory approach, specifically the Rasch mathematical model, to develop the short measure. Rasch is a mathematical process that converts ordinal data (such as the responses in the HOOS-PS of none, mild, moderate, severe, extreme) to interval level data .
Ordinal data have logical order but the distances between question responses like ’none’ and ’mild’ and ’mild’ and ‘moderate’ may be different. In contrast, interval level data has order and the distances between scores are equal. The advantage of interval level data is that we can accurately quantify physical disability to be able to compare individuals and groups and to know how much they have changed.
On a measure that has interval level scaling properties, we know that people who change from 10 to 20 points and those who change from 60 to 70 points have all changed exactly ten points and that this ten points is equal along the scale range. On an ordinal scale, although the questions are often summed, it is often assumed but not confirmed that this scenario would represent 10 points that are of equal distance along the scale.
The most basic form of the Rasch model, based on a dichotomous response scale, is that the probability of a person endorsing an item is a logistic function of the difference between the person’s ability and the difficulty of the item. This can be expressed as a logit model:
where ln is the logarithm function, p is the probability of person n endorsing item i, φn is the level of functional ability of person n, and bi is the difficulty of item i. The item and person estimates are expressed as logits which allows for linear transformation of the raw score. As an item estimate is based on responses to the other items, the model is able to accommodate missing responses to an item for a given respondent. In the case of multiple response options such as with the HOOS, item and person score estimates are similarly estimated by extension of the model [3,4].
The Rasch model provides a number of advantages over classical test theory methods. Specifically, it provides scores that have the requisite interval level measurement properties for inferential statistical procedures; estimates place items in a hierarchy of difficulty such that range of difficulty represented by the measure is known; and, the standard error of an item is independent of the SE of other items such there is improved accuracy and stability of the performance of the items across different samples [3,4].
The final items included in the short measure were based on the data from the samples and countries described below meeting the criteria for fit to the Rasch model .
What is the purpose of the HOOS-PS?
For what populations is the HOOS-PS relevant?
Individuals were from community and clinical samples and were from Sweden, Canada, and the Eurohip study which represents data from Austria, Finland, France, Germany, Hungary, Iceland, Italy, Poland, Spain, Sweden, Switzerland and the United Kingdom. These individuals ranged from 19 to 96 years.
What are the measurement properties of the HOOS-PS?
How do I use and score the measure?
What studies are currently using the HOOS-PS?
This study is being conducted in Australia, Canada, the Czech Republic, France, Germany, Italy, the Netherlands, Spain, Sweden, Britain and the United States. Additionally, the HOOS-PS is being evaluated in people undergoing total joint replacement to further test its reliability, validity and responsiveness in Canada.