Blinding in Clinical Trials

7. Double Blind

A clinical trial design in which neither the participants nor investigators know which participants are receiving the experimental medicine and which are receiving a placebo (or comparator therapy). Double-blind trials are thought to produce objective results, since the expectations of the investigator and the participant do not affect the outcome. Also called double-masked trial.

  • Considered best-controlled trial design.
  • Decreased chance of preconceived notions or physical cues (e.g. the placebo effect, observer bias, experimenter's bias) to distort the results.
  • The key that identifies the participants and which group they belonged to is kept by a third party, and is not revealed to the researchers until the study is over.
  • Should be used whenever possible.
In trials in studies comparing two active compounds (test medicine and comparator) and when the two treatments cannot be made identical, double dummy is a technique for retaining the blind. Supplies are prepared for Treatment A (active and indistinguishable placebo) and for Treatment B (active and indistinguishable placebo). Participants then take two sets of treatment; either A (active) and B (placebo), or A (placebo) and B (active).

For example, if we want to compare two medicines, one presented as green tablets and one as pink capsules, we could also supply green placebo tablets and pink placebo capsules so that both groups of patients would take one green tablet and one pink capsule.