Photo credit: ArturVerkhovetskiy/Deposit Photos
Here's a simple headline to work with: "Kids who eat more fruits, vegetables have better mental health". The story appears on a mega-site called studyfinds.org, which provides multiple summaries of science research.
a) What are the variables in this headline?
b) Does the headline make a frequency, association, or causal claim (and why?)
c) Before reading on, do you think that the variables in this study were measured or manipulated?
Now here are some details about the study:
“While the links between nutrition and physical health are well understood, until now, not much has been known about whether nutrition plays a part in children’s emotional well-being,” says says lead researcher Ailsa Welch ... “So, we set out to investigate the association between dietary choices and mental wellbeing among schoolchildren.”
Researchers studied data from more than 8,500 children at 50 schools in England.
d) Based on this description, can you tell how the "diet" and
"well being" variables were probably operationalized--Self-report? Observational? or Physiological? (If not, what would you guess?)
Here are some details of the results:
The team looked at the association between nutritional factors and mental well-being and took into account other factors that might have an impact, such as adverse childhood experiences and home situations. “We found that eating well was associated with better mental well-being in children. Among secondary school children, in particular, there was a really strong link between eating a nutritious diet, packed with fruit and vegetables, and having better mental well-being,” explains co-author Dr. Richard Hayhoe....
e) Start with this quote: "there was a really strong link between eating a nutritious diet, packed with fruit and vegetables, and having better mental well-being,” Sketch a scatterplot of this relationship, labeling your axes mindfully.
f) Now consider this statement: The team looked at the association between nutritional factors and mental well-being and took into account other factors that might have an impact, such as adverse childhood experiences and home situations. The phrase "took into account" should alert you that regression was used. In this regression, what was the criterion (dependent) variable? What were the predictor (independent) variables? (hint: there were at least three predictor variables).
g) Suppose someone wanted to claim, based on this study, that eating fruits and vegetables causes better mental well-being. What are some internal validity problems with that claim? Come up with one or two possible C-variables that could be responsible for this relationship, and explain how your C variable goes with both A (eating fruits and veg) and B (higher mental well-being).
h) When the researcher reported that "among secondary school children, in particular, there was a really strong link between eating a nutritious diet..and having better mental well-being," he is suggesting a moderator. The term, "in particular," suggests that the relationship between diet and well-being was stronger for secondary school children than it was for younger children. Sketch what this moderator would look like, either using two scatterplots (one for each age subgroup, kind of like Figure 8.18) or making a table, similar to Table 8.5.
i) If you like, think about mediators. Let's say that the researchers have a theory about why eating more fruit and veg is associated with better mental well-being. What might mediate this relationship? That is, why might eating fruit/veg lead to better mental well-being? Think of a mediator and draw a small diagram of it here, like Figure 9.12.
(Please keep in mind that a mediator relationship is a causal relationship, and that can't really be supported by this correlational study. However, that does not stop us from proposing some potential mediators that we can test in a future study.)
