Minimal important difference: Values for the Foot Health Status Questionnaire, Foot Function Index and Visual Analogue Scale
Introduction
It is common for clinicians and researchers to view statistical significance as being equal to clinical significance [1]. Any treatment that is demonstrated to be ‘significantly’ different (i.e. better) from a statistical standpoint to another treatment is commonly then suggested to be more effective and accordingly, incorporated into clinical practice.
Statistical significance only refers to the probability that the result of an analysis between two treatments is truly different. Thus, a significant finding provides us only with a degree of confidence that the result is not a chance finding [2]. However, the mere existence of a statistically significant treatment effect does not always mean the effect is large, or even clinically worthwhile. That is, the effect may not be important to patients [3]. There is a very real chance, therefore, that a statistically significant result might be so small as to be meaningless to patients because they are unable to detect it. The problem of equating statistical significance with clinical importance is that it ignores the possibility that a statistically significant result may not always be clinically relevant, or equally, a clinically important finding may not always be statistically significant [1], [4], [5].
The most effective method of examining a study's results for clinical importance is by comparing the results against an already established minimal important difference (MID) for the outcome measure being implemented [1]. A minimal important difference has been defined “as the smallest difference in score in the domain of interest which patients perceive as beneficial and which would mandate, in the absence of troublesome side effects and excessive cost, a change in patients’ management” [6, p. 408]. This value was formerly known as a minimal clinically important difference (MCID), however deletion of the word ‘clinical’ was made to remove the focus on the clinical context rather than the patient's experience [7].
Once established, the minimal important difference for an outcome measure is not only useful for interpreting trial results, it is also important for sample size calculations [1], [8]. An appropriate sample size ensures sufficient power to detect, using statistical tests, a clinically worthwhile result (i.e. a minimal important difference) if one indeed exists. This point is fundamental to good clinical trial design—without an appropriate, prospective sample size calculation the chance of statistically invalid findings will be high due to a lack of power.
This study, therefore, aimed to investigate the minimal important difference of three commonly used outcome measures used in foot-related research: the Foot Health Status Questionnaire, the Foot Function Index and the Visual Analogue Scale.
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Methods
Data for this study were taken from two trials evaluating conservative treatments for plantar fasciitis. Both trials used similar methodology – the same general protocol and outcome measures – although one trial evaluated a short-term treatment (outcomes measured over 2 weeks) and the other a more long-term intervention (outcomes measured over 12 months). The long-term trial ran from 1999 to 2001 and the short-term trial in 2002. Findings relating to the effectiveness of the treatments
Results
A total of 175 participants were recruited into the two trials. The mean change scores from all time points for both trials were combined. To appreciate the number of individual participant responses that were analysed, the total pooled number of data entries for each domain of the outcome measure are presented in Table 1.
The mean ‘no change’ and ‘a little change’ scores are presented with the minimal important differences for all outcomes in Table 2, Table 3, Table 4. The ‘no change’ score
Discussion
When evaluating the effectiveness of interventions in trials relating to the foot, pain is a common outcome measure. While pain is an important outcome, it does not take into account the effect a disorder has on the patient's life in general. Recently, outcome measures have begun to focus on health status and health-related quality of life, where a broader interpretation of the impact of a condition on a person's life is taken into account [21]. The Foot Health Status Questionnaire and Foot
Conclusion
This research project calculated minimal important differences for the Foot Health Status Questionnaire, the Foot Function Index and the Visual Analogue Scale. Our findings will assist with interpreting results from clinical trials that have used these outcome measures to evaluate the effectiveness of interventions, particularly for the treatment of plantar fasciitis. Further, with the limitations outlined in mind, researchers can cautiously use these minimal important differences to assist in
Conflict of interest
The authors have no financial or personal relationships with other people or organizations that could inappropriately influence (bias) this work.
Acknowledgement
We thank Anne-Maree Keenan and Anthony Redmond for their role in this project.
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