You are interested in seeing whether an intervention you have designed (pointing out both the height and width of a glass) assists children of 5 years old to pass a conservation of liquid task. You test 60 5-year-olds from kindergarten classes in two different schools on opposite sides of town. The 30 5-year-olds from school A receive the intervention and the 30 5-year-olds from school B receive no such intervention. After one week of training, you find that a much greater percentage of children in the intervention group passed the conservation task than those in the no intervention group. You conclude that the intervention was a success. In the scenario above, what is the confounding variable? the two different schools in your study (i.e., school A versus B) intervention (presence versus absence of an intervention) performance on the conservation of liquid task age of child