Selective courses such as Honors or AP (Advanced Placement) wereexcluded as were three sets of letters where all students in one class wroteon the same topic.
While these students were excluded from statistical extrapolation, this is evidence there was not control over how participation was assigned or how the opportunity was given. Students in other classes might have had limited options for topics, might have been guided or impacted by preceding lessons on civics, might have had letters read prior to submission, impacting a student's sense of freedom to write a true and preferred opinion.