Question

zlatan is a dietitian who is studying the effects of a protein supplement on the muscle mass of his clients. however, at the end of his 8-week study, he concludes that his experiment lacks internal validity. what is a possible reason for this?
zlatan realized that he did not define the experimental variables in normal, realistic ways.
one cannot establish a cause-and-effect relationship in 8 weeks.
he calculated a ( p < 0.05 ).
he did not control other aspects of his clients diets.

Explanation:

To determine why the experiment lacks internal validity, we analyze each option:

• Option 1: Defining variables unrealistically affects external validity (generalizability), not internal validity (cause - effect). So this is incorrect.
• Option 2: The time frame (8 weeks) alone doesn't determine if cause - effect can be established. It depends on the experiment, so this is wrong.
• Option 3: A \( p < 0.05 \) indicates statistical significance, which is related to the likelihood of results being due to chance, not internal validity. So this is incorrect.
• Option 4: Internal validity is about establishing cause - effect. If other aspects of clients' diets (confounding variables) are not controlled, it's hard to say the protein supplement (independent variable) caused changes in muscle mass (dependent variable). This is a reason for lack of internal validity.

Answer:

D. He did not control other aspects of his clients' diets.