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There is a common misperception that a statistically significant result in clinical research equates to a meaningful or worthwhile outcome for patients. However, not all statistically significant findings are considered important by patients.
This research project aimed to calculate the minimal important difference for commonly used outcome measures in research related to the feet.
Data from 175 participants from two trials that evaluated conservative interventions for plantar fasciitis were used to determine minimal important differences for the following outcome measures: the Foot Health Status Questionnaire, the original Foot Function Index and a Visual Analogue Scale used to measure pain. The mean change in the outcome measure was compared to a simple global question of improvement. The amount of change in the outcome measure that related to “a little change” in the global improvement question represented the minimal important difference.
For the Foot Health Status Questionnaire the following minimal important differences were found: 14 for pain, 7 for function and 9 for general foot health. Similar results were found for the Foot Function Index: 12 for pain, 7 for disability and 7 for Total Foot Function Index. The minimal important difference for the Visual Analogue Scale was 9 mm.
Our findings can be used to help interpret results from clinical trials that have used these outcome measures to evaluate the effectiveness of interventions, particular for the treatment of plantar fasciitis. Researchers can also utilise these minimal important differences to assist in prospective sample size calculations for clinical trials, although caution is required for trials on disorders other than plantar fasciitis
The Visual Analogue Scale (VAS) and the Foot Health Status Questionnaire (FHSQ) are two commonly used outcome measures for evaluating foot health. This study aimed to calculate the Minimal Important Difference (MID) of the VAS and the FHSQ.
184 participants with plantar heel pain were recruited from the general public to take part in two randomised trials (92 participants in each trial) that studied the effectiveness of two conservative interventions for plantar heel pain. Data from these participants were used to calculate the MIDs of the VAS and the FHSQ. An anchor-based method was used to calculate the MIDs. Two distinct types of pain were investigated for the VAS: average pain and first-step pain. All four domains of the FHSQ were investigated: foot pain, foot function, footwear and general foot health.
The MID for the VAS using the anchor-based approach was -8 mm (95% CI: -12 to -4) for average pain and -19 mm (95% CI: -25 to -13) for first-step pain on the 100 mm VAS. The MID for the FHSQ was 13 points (95% CI: 6 to 19) for pain and 7 points (95% CI: 1 to 13) for function. The MID for the footwear domain of the FHSQ was -2 points (95% CI: -8 to 4) and 0 points (95% CI: -7 to 6) for the general foot health domain of the FHSQ.
The results of this study provide additional evidence for MID values of the VAS and the FHSQ for plantar heel pain. This is important for clinicians and researchers as it provides a greater understanding of how much improvement is required by a patient before a minimal, worthwhile change is experienced. The calculated MIDs will also assist researchers with prospective sample size calculations.