Cumulative proportion of individuals who have not had a license action since receiving primary certification for physicians in the 1994 to 1999 cohort who did not voluntarily participate in maintenance of certification (MOC; n = 8,441, red line), those in the 1994 to 1999 cohort who voluntarily participated in MOC (n = 953, blue line), and those in the 2000 to 2005 cohort who were required to participate in MOC (n = 6,023, black line), as estimated with the Kaplan–Meier method. Shown below the abscissa are the numbers of physicians at risk for each category.