  Clinical trials traditionally employ blinding as a design mechanism to reduce
the influence of placebo effects. In practice, however, it can be difficult or
impossible to blind study participants and unblinded trials are common in
medical research. Here we show how instrumental variables can be used to
quantify and disentangle treatment and placebo effects in randomized clinical
trials comparing control and active treatments in the presence of confounders.
The key idea is to use randomization to separately manipulate treatment
assignment and psychological encouragement messages that increase the
participants' desire for improved symptoms. The proposed approach is able to
improve the estimation of treatment effects in blinded studies and, most
importantly, opens the doors to account for placebo effects in unblinded
trials.
