PURPOSE: To assess the minimal important difference (MID) of the Symptom Status Index (SSI) using the distribution-based standard error of measurement (SEM) method.
METHODS: The SSI is a diary-based index comprised of 4 VAS items: shortness of breath, tiredness, activity limitation, and frustration with symptoms. It has been shown to be a reliable, valid, and a responsive tool in COPD population. Individual item scores are summed to obtain the SSI total score. Patients record the severity of their symptom status on a 0mm (none/not at all) to 100mm (worst it has ever been/as bad as it can be) VAS. The SEM method was used to determine the MID because of its sample independent properties. This results from simultaneous incorporation of sample reliability and variability in its determination. The one-SEM criterion has been shown to represent MID and was used in this analysis as MID criterion. Data from 3 sets of twin trials (Study sets A, B, & C) in patients with COPD was used in this analysis. Internal consistency reliability (Cronbach's α) and the standard deviation of the summated SSI scores, which are required for computation of the SEM, were determined for the SSI.
RESULTS: SEM based MID for SSI based on 3 study sets are illustrated in the Table.
CONCLUSION: Consistent results across the 3 sets of trials point to about 5 to 6 unit change on the SSI (0 to 100 scale) as MID.
