Minimal important difference: Values for the Foot Health Status Questionnaire, Foot Function Index and Visual Analogue Scale

Abstract

Background 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. Objective This research project aimed to calculate the minimal important difference for commonly used outcome measures in research related to the feet. Methods 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. Results 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. Conclusion 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.