An example of a posttest only nonequivalent groups design is evaluating a new fraction-teaching strategy by implementing it in one third-grade classroom (the treatment group) and comparing the final math scores to another classroom taught with traditional methods (the control group). Because the researcher did not randomly assign students to these classes, preexisting factors—such as varying student motivation levels, differing classroom environments, or distinct teacher disciplinary styles—serve as confounding variables that could independently explain any outcome differences.