Adding a Year versus Subtracting a Year
It should be clear that a valid control group is not identical to "doing everything the same except for the change that is being tested". The placebo effect is taken into consideration in clinical studies, for example, by an active control. All patients receive medicine, some receive the drug being tested while others receive a similar pill but does not contain the active ingredient being tested. In some cases, these tests are done double-blind - even the medical practitioner providing the medicine does not know who gets what.
Unfortunately, research in education does not normally employ such rigor and one therefore has to be careful in reading reports on education. Take, for example, the years in high school:
The following is a screen capture of a news article from the Texas Tribune:
Journal of Educational Research:
With more students pursuing university, it becomes essential for high schools to provide training that maximizes their graduates’ success. There is debate over whether an extra year of high school better prepares students for university. The authors used a nationally representative survey to contrast academic and employment outcomes between high school graduates of Grade 12 and Grade 13. Results suggest that Grade 13 graduates obtain higher grades in high school, are more likely to pursue university, and are less likely to be employed full time compared with Grade 12 graduates. Among students enrolled in university, Grade 13 graduates report higher grades in university and satisfaction with their program and do not transfer out of their programs compared to Grade 12 graduates. These findings highlight the importance of an additional year of high school to produce better prepared and more satisfied graduates.
Confusing? It seems so. The point is neither one is based on a study that has a valid active control. The additional years in high school unfortunately do not subscribe easily to a controlled study. What is inside those years matters. What happens in a year matters. Thus, the Dallas school district in Texas simply addresses this issue from the point of view of finance and priorities.