While there are many types of headaches, many of them come from stress (both physical and emotional stress). Usually, people with headaches will also notice neck tightness and discomfort. Sadly, though, most will never ever notice that their headaches are often deeply rooted in the posture of their low back and how their head and neck are working so hard not to slump forward.
While it is just a guess, my estimate is that over 80 percent of people that come in with a headache leave without one. For the remainder, 15 percent are largely decreased and the remaining 5 percent of headaches are either not structural in nature or require multiple visits to get to the root cause.