Problem: Zlatan is a dietitian who is studying the effects of a protein supplement on the muscle mass of his clients. However, at the end of his 8-week study, Zlatan concludes that the experiment lacks internal validity. What is a possible reason for this? Need Assignment Help? One cannot establish a cause-and-effect relationship in 8 weeks. Zlatan realized that he did not define the experimental variables accurately. Zlatan calculated a p < .05. Zlatan did not control other aspects of the clients' diets.