Haas M, et al. - The purpose of researchers was to identify the dose-response relationship between visits for spinal manipulative therapy (SMT) and chronic cervicogenic headache (CGH) outcomes. They also evaluated the effectiveness of SMT by comparison with a light massage control. Between SMT visits and days with CGH, there was a linear dose-response relationship. Results suggested that for the highest and most effective dose of 18 SMT visits, CGH days were reduced by half, and about 3 more days per month than for the light-massage control.