Let’s say that study researchers created two truly random groups of participants. One group received fish oil and the other received olive oil. But, participants could taste the difference between the fish and the olive oils.
When this happens, the researchers have failed to properly “mask” or “blind” their participants.
Masking or Blinding describes how researchers hide who is receiving the treatment and who is receiving a placebo/alternative treatment. Blinding is important for participants as well as anyone who treats or measures outcomes in them.