In the experimental method, all extraneous variables are controlled. All other variables are kept constant, either through experimental control or randomization. If the scores of the groups are different, can conclude that it was caused by the independent variable (because that was the only difference between the groups). Good experimental design eliminates possible confounding that results in possible alternative explanations, because only by eliminating competing, alternative explanations can we draw a causal relationship from the independent variable. Internal validity when the results of an experiment can confidently be attributed to the independent variable (and not any alternate explanations). The simplest experimental design has two variables the independent and dependant variables. The independent has two levels the control group and the experimental group. Researchers make every effort to ensure the only difference between groups is the manipulated variable remember experiments involve control over extraneous variable through keeping such variables constant (control group) or by randomization.