Annual incidence of psoriatic arthritis by age and sex. Shown is the annual incidence (per 100,000) of psoriatic arthritis by age and sex (1 January 1970 to 31 December 1999; Olmsted County, Minnesota). Broken lines represent smoothed incidence curves obtained using smoothing splines. Reproduced from  with permission.
Gabriel and Michaud Arthritis Research & Therapy 2009 11:229 doi:10.1186/ar2669