What is the prevalence in the general population?
From Wikipedia, with dysthymia being another word for "chronic depression":
Globally dysthymia occurs in about 105 million people a year (1.5% of the population). It is slightly more common in women (1.8%) than in men (1.3%). The lifetime prevalence rate of dysthymia in community settings appears to range from 3 to 6% in the United States. However, in primary care settings the rate is higher ranging from 5 to 15 percent. United States prevalence rates tend to be somewhat higher than rates in other countries.