True regarding double blinding placebo controlled clinical trial:
Some patients are not treated
The clinician does not know which treatment is given to the patient
50% of the patients do not know what drug they receive
Everybody receives both the drugs
(Refer: K. Park’s Textbook of Preventive and Social medicine, 24th edition, pg no: 78)
Bias

Any systematic error in an epidemiological study occurring during data collection, compilation, analysis and interpretation
Types of bias