suppose that as a topic for a research study you are interested in brain in jury that may result from repeated blows to the head such as those suffered by boxers and soccer players for obvious ethical reasons (physical harm) you could not plan a study that involved injuring people brains to examine the effects however you could compare two preexisting groups for example a group of soccer players who are regularly hit on the head with soccer balls and a group of swimmers who are also athletes but are not routinely hit in the head (see down & Abwender 2002 for a sample study)