The main aim of this study is to investigate the biased effects which affect scientific researches; although most of the researchers ignore them, and make criticism of the experimental works in the educational field, based on this viewpoint. For this reason, a quasi-experimental process has been carried out with five different student teachers groups, to see the John Henry effect which is one of those biased artifacts. At the end of the whole process, it was seen that the John Henry effect plays a great role on students' achievements. At the end of the study, experimental works were discussed on the basis of those artifacts and some recommendations were presented on how we can reduce the effects of those artifacts or how we can use them to get benefit for our experimental works. (Contains 3 figures and 7 tables.)