Evaluation of the Effectiveness of the Tap Test by Combining the Use of Functional Gait Assessment and Global Rating of Change

A number of studies have computed the minimally important difference (MID) for health-related quality of life instruments. To determine whether there is consistency in the magnitude of MID estimates from different instruments. We conducted a systematic review of the literature to identify studies that computed an MID and contained sufficient information to compute an effect size (ES). Thirty-eight studies fulfilled the criteria, resulting in 62 ESs. For all but 6 studies, the MID estimates were close to one half a SD (mean = 0.495, SD = 0.155). There was no consistent relationship with factors such as disease-specific or generic instrument or the number of response options. Negative changes were not associated with larger ESs. Population-based estimation procedures and brief follow-up were associated with smaller ESs, and acute conditions with larger ESs. An explanation for this consistency is that research in psychology has shown that the limit of people's ability to discriminate over a wide range of tasks is approximately 1 part in 7, which is very close to half a SD. In most circumstances, the threshold of discrimination for changes in health-related quality of life for chronic diseases appears to be approximately half a SD.

Most clinicians ask their patients to rate whether their health condition has improved or deteriorated over time and then use this information to guide management decisions. Many studies also use patient-rated change as an outcome measure to determine the efficacy of a particular treatment. Global rating of change (GRC) scales provide a method of obtaining this information in a manner that is quick, flexible, and efficient. As with any outcome measure, however, meaningful interpretation of results can only be undertaken with due consideration of the clinimetric properties, strengths, and weaknesses of the instrument. The purpose of this article is to summarize this information to assist appropriate interpretation of the GRC results and to provide evidence-informed advice to guide design and administration of GRC scales. These considerations are relevant and applicable to the use of GRC scales both in the clinic and in research.