The causal headlines are bearing some great news--
The press is buzzing about a recent study from Tel Aviv University. Here's the abstract from the journal article. The researchers randomly assigned people to one of two low-calorie diets. Women ate 1400 calories on both diets; men ate 1600 calories. The one difference was that on the first diet, breakfast was a low-carb 300 calorie meal. On the other diet, breakfast was high carbohydrate, high protein meal that included dessert such as cookies or cake.
Unlike those in the low-carb breakfast group, the dieters who ate dessert at breakast were more likely to keep their weight off during the follow up period. In fact, they continued to lose an average of 13 extra pounds, while the other group gained all their weight back.
a.) What do you think--does the study described in the abstract support the causal statement that "eating dessert at breakfast can help you lose weight"? Consider the research design and go through the three causal rules in this example.
b.) What causal claim could you make about this study?
a) The study is an experiment, so it can potentially suport a causal claim. The results did show covariance: dieters who ate dessert at breakfast did lose more weight than dieters who did not. Because the dessert aspect of the diet was manipulated by the researchers, we know that the dessert came first and the weight loss came later (therefore, there is temporal precedence). What about internal validity? There's a potential problem here, because the dieters who ate dessert for breakfast also had a high-protein breakfast. The comparison group did not eat dessert, but they also ate a low-carb breakfast, too. So we don't know if the weight loss was caused by the dessert, by the high protein, or by some combination of these two.
b) Because the study appeared to have manipulated two variables, journalists would be more accurate if they said that "eating a high-protein, high carb breakfast that includes dessert can help dieters lose weight."