Use more than one control group. Try a different experimental group. Change the hypothesis if it is incorrect. Repeat the procedure exactly the same each time.
I gave my students this multiple choice question: Which is the most reliable way to get unbiased results when conducting a controlled experiment?

Only one answer is correct among these 4 choices:
- Use more than one control group.
- Try a different experimental group.
- Change the hypothesis if it is incorrect.
- Repeat the procedure exactly the same each time.

Could you tell me which one is correct?

Repeat the procedure exactly the same each time.