This recent report from the New York Times focuses on California's high school students. About five years ago, the state banned junk foods like soda, chips, and candy from high schools in the state. Now a study has reported that California students eat fewer calories than students in other states.
The study found that California high school students consumed on average nearly 160 calories fewer per day than students in other states, the equivalent of cutting out a small bag of potato chips.
To study the effect of this policy, the researchers examined data from the Centers for Disease Control and Prevention on the eating habits of high school students in California, comparing it with data on students from 14 states that did not have nutrition standards for vending machine snacks and other foods sold outside of school lunches and other meal plans.
a) This study's design seems to be a quasi-experiment. What makes it so?
b) Sketch a graph of the study and its results.
d) Overall, do you think the evidence presented here supports the claim that banning junk food in schools can help kids eat fewer calories?
a) This is a quasi experiment--probably a nonequivalent groups, post-test only design. It is a quasi experiment because the independent variable (being in a state that bans junk food in school) is not manipulated. Students cannot be randomly assigned to live in a state with a junk food ban (i.e., CA) or to a state without junk food ban (i.e., the other 14 states).
b) Your sketch would probably have "Calories consumed daily" on the y-axis and "state" on the x-axis. On the x-axis you could plot all 15 states separately (showing CA with the lowest calorie consumption), or you could plot California and an average of the remaining 14 states (showing CA with the lowest calorie consumption).
c) We might worry that compared to the other 14 states in the study, CA students come disproportionately from one ethnic background (for example, there are more Hispanic students there), or that CA and the comparison states may also have different requirements for physical education in school, which might affect eating habits. To find out how the researchers controlled for these factors, you could check the original source in the Archives of Pediatrics and Adolescent Medicine.
d) If, after reading the original report, you were convinced that the junk food ban was the only "active ingredient" separating CA from the 14 comparison states, then you might well be convinced that the junk food ban is playing a role in controlling child and adolescent obesity.