Students with Low, Average, and High Scores on a Commercial

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:

A) reactive effects of the experimental procedures
B) contemporary history
C) pretesting procedures
D) statistical regression