Four methods of estimating the minimal important difference score were compared to establish a clinically significant change in Headache Impact Test.

OBJECTIVE: To estimate the smallest decrease in Headache Impact Test (HIT) scores that reflects meaningful clinical change among patients with chronic daily headache (CDH). STUDY DESIGN AND SETTING: We applied four methods of estimating the minimum important difference (MID) to data from 71 patients with CDH who participated in a clinical trial. The HIT was administered at baseline and at the 6-week follow-up assessment. Patients were considered to have experienced meaningful improvement if they reported that their headache condition was "somewhat better" or "much better" at the 6-week follow-up. RESULTS: Mean HIT scores at baseline and 6 weeks for all patients were 64.5 (standard deviation SD = 6.0) and 62.6 (SD = 5.7), respectively. HIT scores decreased 3.7 (SD = 4.4) and 1.4 (SD = 3.6) units, respectively, among patients who reported "somewhat better" change and those who reported no change at 6 weeks. Estimates of the MID of the HIT ranged from -2.7 to -2.3. CONCLUSIONS: The method that we judge to be most valid estimated the MID of the HIT at -2.3 units (95% confidence interval = -4.3, -0.3). This suggests that a between-group difference in HIT change scores of 2.3 units over time among patients with CDH reflects improvement in patients' headache condition that may be considered clinically significant.