What is new?
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Global Perceived Effect (GPE) scales can be reliably rated by patients with musculoskeletal conditions.
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Patients have difficulty taking their baseline status into account when scoring the GPE, and ratings are very strongly influenced by their current health status.
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The influence of current status may increase with longer transition time.
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GPE ratings may not offer an accurate measure of change as transition time stretches into months.
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GPE ratings may be unsuitable for use as external criteria of change when determining minimally important change and responsiveness of other instruments.
The measurement of complex constructs, such as recovery, pain, and disability, is a difficult process, both for clinicians and researchers. Therefore, establishing the reliability and validity of instruments that aim to measure these constructs is necessary to sensibly interpret the findings of clinical studies. Similarly, clinicians need to understand the strengths and shortcomings of the outcome measures they use to inform appropriate clinical practice [1].
In clinical practice, measurement of patient-rated recovery often takes the form of the question: Are you feeling better (or worse)? Or, to what extent have you improved (or deteriorated) since last time? This type of rating of perceived recovery is a “transition scale” or Global Perceived Effect (GPE) scale. The GPE scale asks the patient to rate, on a numerical scale, how much their condition has improved or deteriorated since some predefined time point. The GPE has several qualities that make it an appealing tool for use in clinical practice and research; being a single question, it is easy and quick to administer and the results are seemingly simple to interpret. Such scales have been recommended for use as a core outcome measure for chronic pain trials [2] and been advocated to increase the relevance of information from clinical trials to clinical practice [3]. From the patient's perspective, the question is intuitively easy to understand and it allows them to rate those aspects of recovery that are most important to them. In addition to measurement of outcome, the GPE is commonly used as an external criterion to test the measurement properties of other outcome measures [4], [5], [6]. In the field of musculoskeletal research, these “other” outcomes are often pain or disability, domains that are assumed to have an important impact on quality of life for these patients.
However, there are several potential limitations of the GPE and relatively little work has been published regarding its reliability and validity. One concern is that the GPE may have low test–retest reliability [7], given that it is a single-item measure. Further, there are validity concerns as there is evidence that patients have difficulty recalling their previous status and their estimates of transition are biased by their current status [8], [9]. It is suggested that this bias would increase as the time period over which the transition span increases, that is, patients would become more likely to confuse change over time with current status as the time interval lengthens [8].
Guyatt et al. [8] conducted an assessment of the measurement properties of the GPE. They began by calculating the correlation between GPE scores and the change in measures of health-related quality of life in subjects with respiratory disorders. They asserted that although a significant correlation is necessary, this alone is insufficient to confirm that the GPE is truly measuring change rather than current status. This is because in some situations current status and change will be highly correlated, in others they will not.
To test the validity of the GPE, Guyatt et al. suggest that not only should there be a strong correlation between the calculated change on another measure and the GPE, arbitrarily set at 0.5, but the pre (baseline) scores on a measure and the post (current) scores should have lower, but approximately equal (size) and opposite (direction) correlations with the GPE. The thesis depends on comparable variances in the pre and post scores and is supported by a mathematical proof in their paper. To explore the matter further, the authors also constructed regression models and entered pre and post scores with GPE as the dependent variable. They reasoned that if the partial regression coefficient for the pre score was significant, it would indicate that baseline scores are taken into account in the calculation of change. The results from the Guyatt et al.'s study are somewhat mixed; in some cases, patients seemed to recall their prior state, even up to 4 weeks, in others they did not. Guyatt et al. suggested that confidence in the utility of transition (GPE) scales is determined by the extent to which correlations between pre and transition and post and transition are similar, where they are very dissimilar the scale may be providing biased information.
The present study was designed to further investigate the measurement properties of the GPE scale in subjects with musculoskeletal conditions. The three aims of the study were as follows. Firstly to establish the test–retest reliability of the scale; secondly to determine to what extent, if any, patients take their baseline status into account when scoring the GPE; and thirdly to determine how much influence the transition time period has on the performance of the scale.