A researcher gives a new vitamin to 60 morning patients and a placebo to 60 evening patients at a clinic. Three months later, vitamin patients report better health on average. However, morning patients also tend to be older retirees who have more time to exercise.
Which variable is confounded with the vitamin treatment?
- A
Random assignment
- B
Measurement of outcome
- C
Sample size
- Dcheck_circle
Time of appointment / age and exercise habits
Explanation
Treatment is perfectly tied to appointment time, which itself relates to age and exercise. Vitamin effects cannot be separated from these other variables. Random assignment would have prevented this confounding.