Four methods of estimating the minimal important difference score were compared to establish a clinically significant...

To estimate the smallest decrease in Headache Impact Test (HIT) scores that reflects meaningful clinical change among patients with chronic daily headache (CDH).

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.

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.

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.