A Researcher Was Interested in Studying the Effects of 60

A researcher was interested in studying the effects of 60 minutes of flexibility training per day on enjoyment of exercise. To recruit participants she put up a poster in the university fitness centre; thirty women subsequently volunteered for her study. Her research design included a pre-test of enjoyment of exercise followed by her intervention in which she asked all participants to participate in a three month long flexibility training program (i.e., each participant completed the 60 minutes of flexibility training on her own in the fitness centre after her regular workout each day). At the end of the training program, twenty-six participants completed a post-test of enjoyment of exercise (four participants dropped out of the study half-way through due to the time commitment required). Based on the changes between the pre-test and post-test on participants' enjoyment scores, the researcher concluded that 60 minutes per day of flexibility training was effective in increasing enjoyment of exercise. Based on the research design described, is the researcher's conclusion justified? Clearly justify your answer with particular reference to internal validity and external validity.