 Although the number of people using public Part C of Medicare has grown dramatically from almost zero since 1998 and is projected to grow dramatically, there are four groups in particular that tend to stick with Medicare Parts A and/or B only and then add a private arrangement.First, it is increasingly common for people to continue to work after joining Medicare at age 65, and they use both Medicare Parts A and/or B (often just Part A) and employer sponsored insurance, and delay deciding between FFS Medicare and capitated-fee Medicare until the employer sponsored insurance is no longer available.Second, many retired people -- particularly people over age 75 or former government employees -- receive group insurance as a benefit of their former employment and these group policies tend not to be Part C plans (although that is changing).