Chronic subdural bleeds develop over a period of days to weeks, often after minor head trauma, though a cause is not identifiable in 50% of patients. They may not be discovered until they present clinically months or years after a head injury. The bleeding from a chronic hematoma is slow and usually stops by itself. Because these hematomas progress slowly, they can more often be stopped before they cause significant damage, especially if they are less than a centimeter wide. In one study, only 22% of patients with chronic subdural bleeds had outcomes worse than "good" or "complete recovery". Chronic subdural hematomas are common in the elderly.