Sigh....even the imprimatur of Harvard Medical School doesn't stop writers from tacking causal headlines onto correlational studies.
In this example, the journalist writes, "The study is only observational [i.e., correlational] and doesn't prove that daily tub bathing staves off heart problems". Yet the headline, "Can hot baths protect your heart?" sounds pretty causal to me.
Here's the journalist's summary of the method:
Researchers analyzed self-reported health and lifestyle information from more than 30,000 middle-aged people in Japan. Participants responded to a questionnaire at the start of the study and were then followed for about 20 years.
And here's the summary of the major results:
Compared with people who didn't take a tub bath more than twice a week, people who took a daily warm or hot bath had a 28% lower risk of cardiovascular disease and a 26% lower risk of stroke.
This article provides a good opportunity to practice the four big validities.
a) Name the three major variables in the study, as included in the journalist's summary.
b) How do you know that the study was correlational? (remember to focus on whether the variables were measured or manipulated)
c) Ask at least one construct validity question about this study. (Hint--start your question with "How well...")
d) Now ask an external validity question about this study. (Reminder--your question should address sampling method, not sample size)
When you consider external validity, it might be relevant to consider that Japan has a well-developed bath culture. Most homes have soaking tubs, and some towns still support a neighborhood sento, or public bathhouse. In addition, many tourist areas and resorts feature beautifully landscaped hot spring baths called onsen. In this study, 2/3 of the participants reported that they bathe in a tub almost daily. What might this bathing culture suggest about the external validity of this study? Would this relationship generalize to a less-bathy culture like the U.S.?
e) What about statistical validity? The authors report two estimates: One estimate concerns the link between bathing and cardiovascular disease (28%), and another concerns the link between bathing and risk of stroke (26%). The journalist didn't provide 95% CIs--but what does the sample size tell you about the 95% CI's of these estimates? (i.e., will they be less precise or more precise?)
f) Now let's walk through why this study can't support a causal claim. This study does have covariance (baths were associated with lower risk of cardiovascular disease and stroke).
What about temporal precedence--what is this, and why does this study achieve it?
And what about internal validity? Come up with a third variable ("C") that is reasonably associated both with taking more baths ("A"), and with having lower rates of disease ("B").
Bonus: The empirical article is open-access and you can read it here. Instructors--it's likely that students would understand this article well enough to read it and critique it.
