"The NBER Bulletin on Aging and Health provides summaries of publications like this. You can sign up to receive the NBER Bulletin on Aging and Health by email. We propose a new classification of experiments that captures the extent to which the experimental design and analysis are linked to economic theory. We then use this system to classify all published field experiments in the five top economics journals from 1975 to 2010. We find that the vast majority of field experiments (68%) are Descriptive studies that lack any explicit model; 18% are Single Model studies that test a single model-based hypothesis; 6% are Competing Models studies that test competing model-based hypotheses; and 8% are Parameter Estimation studies that estimate structural parameters in a completely specified model. Using the same system to classify laboratory experiments published over the same period, we find that economic theory has played a more central role in the laboratory than in the field. Finally, we discuss in detail three sets of field experiments, on gift exchange, on charitable giving, and on negative income tax, that illustrate both the benefits and the potential costs of a tighter link between experimental design and theoretical underpinnings"--National Bureau of Economic Research web site.