in Abhijit Banerjee: Rigour In Social Policy:
"Politicians, journalists, donors--they all want stories of instant success. But it pays to research ideas thoroughly, and experiment in different environments before extrapolating social welfare schemes to a widespread audience."
from the series 25 Ideas To Change The World
P.S. See also the experiment in M. Rajshekhar's report Chhattisgarh: How workers improved healthcare for 18 million people.