True regarding double blinding placebo controlled clinical trial:
(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
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