Dr. Taylor is testing a new antidepressant drug therapy for individuals diagnosed with clinical depression. She is supposed to withhold information from the participants on whether or not they are receiving a pill with the active drug or a pill with no active ingredient. She thinks this is too hard to do and tells the individuals which pill they are receiving. This lack of control in her study could possibly lead to WHICH ONE OF THE FOLLOWING?

• randomized control trial
• placebo effect
• lack of external validity
• meta-analysis