The study found an association between dog ownership and longevity. Which third variables were controlled for? Which third variables might still be responsible for the relationship? Photo: Shutterstock
I can't resist blogging about research on dogs. Here's a story on NPR that summarized several studies that have tested the correlation between longevity and dog ownership. The journalist introduces the topic this way:
Dogs and humans that interact with one another get a jolt of oxytocin, the so-called "cuddle hormone." And, if you get to look at dogs and hug them every day, you just might live longer than people who don't have to clean animal hair off their clothes, according to a pair of studies out this month.
Here is a summary of one of the studies:
[researchers] looked over a 12-year period at dog owners in Sweden who have cardiovascular disease. They adjusted for the kinds of things we know affect cardiovascular health — age, demographics, socioeconomic status, marital status, number of children at home — and even after adjusting for all of that, they found a benefit of dog ownership [on longevity].
....they followed over 100,000 people and found a 21% reduction in deaths from any cause.
The study above used multiple regression.
a) What was the dependent (criterion) variable in this study? What were the predictor variables?
b) Sketch a little regression table with results that would be consistent with what is mentioned above. Put the criterion variable at the top, and list the predictor variables. Which betas would be significant here?
c) The researchers controlled for age, demographics, SES, marital status, and children--but regression doesn't control for all third variables. What else could be associated with both dog ownership and longevity?
d) They mention a "21% reduction in deaths from any cause." Which of the four big validities is this concerning?
One of the experts who was interviewed for this story said:
In particular, I found it very convincing and striking that the benefits seems to be larger among individuals who live alone compared to multiperson households. That suggests the companionship of a dog is possibly very important to their heart health.
e) This is a moderator! Identify the variables in this relationship and complete this sentence: ____ moderates the relationship between ____ and _____.
In part of the article, the journalist presents quotes from an expert, who seems to equivocate on the causality issue:
"When you look at the big picture and look at all the evidence around dog ownership and cardiovascular health, it's pretty clear the signal is real and likely causal," he says.
In other words, he's convinced getting a dog improves your health — especially for those with heart disease.
The research is not definitive, though, Kazi adds. "They're not randomized trials, the gold standard for what we would do to evaluate a new drug," he says.
f) It is frustrating to read that the expert believes the "signal is real and likely causal" and then admits that "it's not definitive." You can set this expert straight. Why can't this correlational study support causation? Apply covariance, temporal precedence, and internal validity. When you address internal validity, check out answer c) above!
Suggested answers
a) The criterion variable is longevity (being alive at the end of the study). The predictor variables are dog ownership, age, demographics, SES, marital status, and children living at home.
b) The table might look like this:
DV: Longevity (or being alive after 12 years)
IV Beta (sig)
Dog ownership .12*
Age -.11
SES .13*
Married .08*
Children at home .07*
d) Current health and mobility might be associated with both dog ownership and longevity. After all, you may need to have a certain level of health and mobility to own a dog, and healthier folks are more likely to be alive at the end of the study.
e) Living status (being alone or married) moderates the relationship between dog ownership and longevity.
f) There is covariance--there is a link between owning a dog and living longer.
There is temporal precedence-- the study took place over several years and dog ownership was measured first, and death rates later.
There is not full internal validity. They controlled for age, demographics, SES, marital status, and children living at home, so they can rule those out. But they did not control for everything. One possibility is that current health and mobility might be associated with both--after all, you may need to be at a certain level of health and mobility to own a dog, and those folks are more likely to be alive at the end of the study.