Question

1.92 Infections Can Lower IQ A headline in June 2015 proclaims "Infections can lower IQ."51 The headline is based on a study in which scientists gave an IQ test to Danish men at age 19. They also ana- lyzed the hospital records of the men and found that 35% of them had been in a hospital with an infec- tion such as an STI or a urinary tract infection. The average IQ score was lower for the men who had an infection than for the men who hadn't. (a) What are the cases in this study? (b) What is the explanatory variable? Is it categori- cal or quantitative? (c) What is the response variable? Is it categorical or quantitative? (d) Does the headline imply causation? (e) Is the study an experiment or an observational study? (f) Is it appropriate to conclude causation in this case?

Answer 1

(a)
The cases in this study are men admitted to the hospital with an infection.

(b)
The explanatory variable is whether the person has an infection or not. It is a categorical variable with values Yes or No.

(c)
The response variable is IQ level. It is an quantitative variable.

(d)
Yes, the headline "Infections can lower IQ" imply causation. It claims that infection in people causes decrease in their IQ level.

(e)
The study is based on what is seen and heard and infer based on the data collected. The researcher has no control over the infection of a given person in the study. There was no mention of applying treatments to a group and recording the effects.
Thus, this is an observational study.

(f)
This is observational study, so it is not appropriate to conclude that infections causes low IQ levels. It only suggests there is association between infection and low IQ.
There can be some other variable which causes infections and low IQ levels at the same time. For example, a severe disease in a person can cause infection as well as affect the brain directly and lower the IQ.