A New York Times article discussed research from the journal Addiction Biology. It tests a theory claiming that tanning is addictive. The sample they used was
…a small group of people from tanning salons who said that they liked to tan at least three times a week and that maintaining a tan was important to them.
The researchers manipulated the type of tanning session that the participants would experience. In one session, the tanners experienced a “normal tanning session” and then answered several questions. Then on another occasion, the participants tanned but the researchers “blocked only the UV light” without telling the tanners of the manipulation.
The results of the study showed that:
The tanners … expressed less desire to tan after the real sessions, indicating they had gotten their fill. But on days when the tanners were unknowingly deprived of the UV rays, their desire to tan after the session remained as high as … before the session began.
In addition, the study found that tanner’s brain activity during the UV session (but not during the sham session) was consistent with areas of the brain that are related to addictions.
a.) What are the independent and dependent variables in this study?
b.) Is this a within groups or independent groups design?
c.) The study used only heavy tanners. What population does this sample represent? Why might the researchers have limited their sample to this group? What validity does this question address?
d.) What causal claim could the researchers be able to make? Does this hold up to the three causal rules?
Suggested answers:
a.) The independent variable was UV exposure, which included two levels, exposure or no exposure to UV rays during the tanning session. One dependent variable was the participant’s desire to tan after the session. Another dependent variable was the participants’ brain activity .
b.) Within groups, repeated measures. We know this was repeated measures because the participants are exposed to each level of the variable. They tan with UV rays and then later tan without UV rays. One thing we don’t know is whether they counterbalanced or accounted for order effects.
c.) Heavy tanners were the experimenter’s “population of interest”, so it makes sense to limit the sample to only heavy tanners. This is an external validity concern.
d.) According to the original NYTimes article, the researchers are testing the idea that there is something addictive about the UV rays that people receive during tanning. Specifically, this study seems to suggest that exposure to UV rays (versus the same tanning experience minus these rays) causes people to feel a reduced need to tan. Does the study have covariance? Yes, they found that a lack of UV ray exposure was associated with a desire to tan; Temporal precedence? Yes. The manipulation comes first and the desire to tan is measured immediately afterwards; Internal Validity? Yes. The article doesn’t address any confounds that they possibly eliminated. Order effects could have an effect on internal validity if all tanners were exposed first to UV rays in one session and not in the next session. Counterbalancing would control for these possible order effects.
(Guest Post by Kate Banford, University of Delaware)