An experiment showed that subjects fed the DASH diet were able to lower their blood pressure by an average of 6.7 points compared to a group fed a "control diet." All meals were prepared by dieticians.

Why were the subjects randomly assigned to the diets instead of letting people pick what they wanted to eat?

Why were the meals prepared by dieticians?

Why did the researches need the control group? If the DASH diet group's blood pressure was lower at the end of the experiment than at the beginning, wouldn't that prove the effectiveness of that diet?

What additional information would you want to know in order to decide whether an average reduction in blood pressure of 6.7 points was statistically significant?