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. Noninteractional effects are found in research that does not require experimenters' interaction with human or animal research subjects. There are three major subtypes of such effects: 1. Scientists observe human behavior, animal behavior, or natural events and record what is observed, but there are errors in what is recorded. These are called observer effects. 2. ...