Can a voucher program boost the learning rates of children living in poverty?
It’s instructive to ask what caused the sea-change in thinking about medical research over the past 50 years. One of the most influential experiences concerned the effectiveness of the Salk vaccine for polio (Meier, 1972). Early studies compared those who received the vaccine to those who did not. The results were discouraging: people receiving the vaccine had polio rates that were as high as those who did not receive it. But there was a problem: Subsequent studies showed that high income families were more likely than low income families to receive the vaccine. Moreover, high income families were also at GREATER RISK of contracting polio. So the early studies were biased against finding a positive effect of the vaccine. A subsequent large scale study in 1954 assigned persons at random to receive the vaccine versus a placebo. The results unmistakably supported the vaccine. Random assignment assured that the two groups had the same risk of contracting polio in the absence of the vaccine
