A school district is comparing two reading programs for high school students reading below grade level. Two very similar schools are used to test the programs, a different one at each school. Students participating in the programs are pretested for their reading abilities and the mean scores are similar at both schools. The reading program implemented at High School A is known to be much more rigorous than the program implemented at High School B. The posttest shows more improvement at High School A, but more of the students dropped out of the program. What threat to internal validity does this pose?

The haitian revolution was the result of a long struggle on the part of the slaves in the french colony of st. domingue, but was also propelled by the free mulattoes who had long faced the trials of being denoted as semi-citizens. this revolt was not unique, as there were several rebellions of its kind against the institution of plantation slavery in the caribbean, but the haitian revolution the most successful. this had a great deal to do with the influence of the french revolution, as it to inspire events in haiti. the haitian revolution would go on to serve as a model for those affected by slavery throughout the world.