An interesting discussion recently emerged (here and here) of a study comparing two agricultural experiments (involving different seeds of cowpeas to farmers in Tanzania) -- one blinded, one unblinded. As Andrew Gelman (Columbia) notes: "Bulte et al. find much different results in the two experiments and
attribute the difference to expectation effects (when people know
they’re receiving an experiment they behave differently); Ozler is
skeptical and attributes the different outcomes to various practical
differences in implementation of the two experiments."