Caspi and colleagues (1997) analyzed results from a large-scale longitudinal study of a sample of children born around 1972 in Dunedin, New Zealand. As one part of their study, the researchers compared the 94 in their sample who were, at age 21, alcohol dependent (clearly alcoholic) versus the 863 who were not alcohol dependent. The researchers compared these two groups in terms of personality test scores from when they were 18 years old. After noting that all results were significant, they reported the following results: Young adults who were alcohol dependent at age 21 scored lower at age 18 on Traditionalism 1d= 0.492, Harm Avoidance 1d= 0.442, Control (d= 0.64), and Social Closeness 1d= 0.402, and higher on Aggression 1d= 0.862, Alienation 1d= 0.662, and Stress Reaction 1d= 0.502.

Explain these results, including why it was especially important for the researchers in this study to give effect sizes, to a person who understands hypothesis testing but has never learned about effect size or power.