Final answer:
It becomes harder to distinguish the effects of the independent variable when study groups are very similar, not adequately controlled for lurking variables, and are not properly randomly assigned.
Step-by-step explanation:
The more similar your study groups are with respect to the variables of interest, the harder it will be to distinguish the effect of the independent variable. When conducting an experiment, researchers aim to have a control group and an experimental group that are identical except for the one variable being tested (the independent variable). If the groups are too similar and do not adequately control for other lurking variables, it becomes difficult to ascertain whether any observed differences in the outcome are due to the independent variable or some other factor.Experimental design controls for lurking variables by randomly assigning subjects to groups, thus ensuring that all potential confounding variables are evenly distributed, and that the only difference between groups is the treatment imposed by the researcher. This is intended to demonstrate a cause-and-effect relationship between the independent variable and the outcome, or the dependent variable.