9.3.8  Multiple body parts II: body parts receive different interventions

A different situation is that in which different parts of the body are randomized to different interventions. ‘Split-mouth’ designs in oral health are of this sort, in which different areas of the mouth are assigned different interventions. These trials have similarities to cross-over trials: whereas in cross-over trials individuals receive multiple treatments at different times, in these trials they receive multiple treatments at different sites. See methods described in Chapter 16 (Section 16.4). It is important to distinguish these studies from those in which participants receive the same intervention at multiple sites (Section 9.3.7).