When scientists conduct experiments, influences and errors occur that affect the results of the experiments. Those influences and errors that occur because of some characteristics of the experimenter or because of something the experimenter did are called experimenter effects. They reduce the validity of the experiment, because the results do not really tell about the hypothesis; they show that the experimenter somehow (usually unwittingly) influenced or changed the results. For that reason, most good researchers look for ways to prevent or minimize experimenter effects. There are two major kinds of experimenter effects: noninteractional and interactional.