A drug company performed a study to test the effectiveness of a new headache medicine. The researchers grouped the patients
into two different groups. One group was given the pills that contained the new medicine. The other group were given pills that
did NOT contain the new medicine.
Why did the researchers NOT tell the patients which pill they were given?
A. to make sure they didn't get angry
B. to prevent errors in data collection
C. to keep people from leaving the study
D. to prevent bias in the data collection