However the DASH has [ 18] excessive internal consistency, with a documented Cronbach Alpha value >0.95 [ 6, 8, 12, 19], the recognized upper limit for 'item redundancy' or the presence of too many items being too similar to enable a valid change to be detected [ 20].
The use of outcome measures which are valid, reliable and sensitive to change is vital when considering the effects of treatment in order to detect valid changes which are greater than measurement error [ 61].
This is a limitation since health outcomes are frequently used for evaluation of clinical trials where valid change scores and access to parametric statistics are required [ 16].
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Mistakes in corroborating conceptual models may result in unnecessary changes to valid systems or in false assurance of invalid systems.
Although the Western Ontario and McMaster University Osteoarthritis Index (WOMAC) is considered the leading outcome measure for patients with osteoarthritis of the lower extremity, recent work has challenged its factorial validity and the physical function subscale's ability to detect valid change when pain and function display different profiles of change.
However, there are two differences between studies that support the WOMAC and those investigations that question its ability to detect valid change.
A third limitation of this study is that it does not provide information concerning the WOMAC-PF's ability to detect valid change if it were administered in alone rather than as part of the full WOMAC.
Our findings support the hypothesis that duplicating activities on the pain and physical function subscales plays an etiologic role in compromising the WOMAC-PF subscale's ability to detect valid change in the presence of discordant change in pain and function.
We believe our study design and methodology will ensure that our results are seen as valid and sufficient basis for changes to clinical practice and systems if these are indicated by our findings.
