D.6 Answers: Overview of internal validity

Answers to exercises in Sect. 6.8.

Answer to Exercise 6.1: Presumably all are extraneous variables, as all are possibly related to the response variable (incidence of depression): That is why the researchers obtained this information. None can be lurking variables, as the researchers measure or observe all of them.

To be a confounding variable, the extraneous variable should be related to both the response variable (incidence of depression) and the explanatory variable (diet quality). As a result, all of the extraneous variables could potentially be confounding variables.

Answer to Exercise 6.2: Response variable: something like ‘risk of developing a cancer of the digestive system.’ Explanatory variable: ‘whether or not the participants drank green tea at least three times a week.’

Lurking variable: ‘health consciousness of the participants,’ because the researchers don’t seem to have measured or observed this.
Answer to Exercise 6.3: Older children would probably be more likely to be smokers, and would be larger in general: age would be a confounding variable. Age is easy to record, and usually is recorded in these types of studies, so probably not a lurking variable. (The age, height and gender of each child is recorded.)