Question

9.31 dont stop exercising! an investigation of the effect of different levels of physical activity was conducted on identical male twins in finland. for each twin pair, although the twins had maintained the same activity level for most of their lives, one twin had significantly reduced activity over the last few years due to work or family pressures. for each pair of twins, body fat percentage, endurance levels, and insulin sensitivity were measured. for the less active twin, the results showed greater body fat, worse endurance, and levels of insulin sensitivity indicating early signs of metabolic disease.24

a. what type of design is being used in this investigation? give the explanatory and response variables.

b. is this an experiment or an observational study? why?

c. the article reports that the measurements were carried out blind. explain what this means and why it is important.

Explanation:

Part a

• Design Type: Matched pairs design (since twin pairs are used).
• Explanatory Variable: Activity level (difference in activity levels between twins in a pair).
• Response Variables: Body fat percentage, endurance levels, insulin sensitivity.

Part b

• Type: Observational study.
• Reason: The researchers did not actively manipulate (assign) the activity levels. The change in activity level (explanatory variable) was due to work or family pressures (natural - occurring change, not researcher - controlled).

Part c

• Meaning of “blind”: The people measuring body fat percentage, endurance levels, and insulin sensitivity did not know which twin was the more active one and which was the less active one.
• Importance: It helps to prevent observer bias. If the measurers knew which twin was less active, they might unconsciously (or consciously) measure or interpret the results (e.g., in the case of endurance levels - maybe pushing the “less active” twin less hard during an endurance test) in a way that would confirm the expected result (that less activity is bad), thus biasing the data.

Answer:

a. Matched pairs design. Explanatory variable: Activity level. Response variables: Body fat percentage, endurance levels, insulin sensitivity.
b. Observational study. Because the researchers did not assign the activity levels (the change in activity was due to external, non - researcher - controlled factors).
c. “Blind” means the measurers didn't know which twin was less active. Important to prevent observer bias.