For example, the research on birth order is quite mixed with some studies showing that first-born children have greater achievement motivation and school success than later-born children and other studies showing no effect. Some of the research shows that the first-born effect disappears when other factors such as family size and educational values are controlled.

If we ignore the mixed research, we will come away with an overly optimistic view of our chance of finding significant effects. We also might fail to include or control for other variables that can affect school achievement.