Students with low, average, and high scores on a commercial reading achievement test were used in a study designed to

determine the effectiveness of a new method of teaching fifth-grade reading. There were no other control or comparison

groups. It was found that the low group made the greatest gain over a six-month period. Concluding that the new method

was most effective with the low group fails to consider some of the possible factors or influences, the most significant of

which is: