Journal of Economic Literature Vol. XLII (December 2004) pp. 1009-1055 Field Experiments GLENN W. HARRISoN and JOHN A. LIST 1. Introduction experimental environment. We do not see the notion of a "sterile environment" as a Iin e e of an pet negative, provided one recognizes its role in Every researcher who behaves as if an exoge- the research discovery process. In one nous variable varies independently of an error sense, that sterility allows us to see in crisp term effectively views their data as coming relief the effects of exogenous treatments on from an experiment. In some cases this belief behavior. However, lab experiments in isola- tion are necessarily limited in relevance for is a matter of a priori judgement; in some cases it is based on auxiliary evidence and predicting field behavior, unless one wants inference; and in some cases it is built into the to insist a priori that those aspects of eco- design of the data collection process. But the nomic behavior under study are perfectly distinction is not always as bright and clear general in a sense that we will explain. Testing that assumption is a recurring difficul- Rather, we see the beauty of lab experi- ty for applied econometricians, and the search ments within a broader context-when they always continues for variables that might bet- are combined with field data, they permit ter qualify as truly exogenous to the process sharper and more convincing inference. under study. Similarly, the growing popularity In search of greater relevance, experi- of explicit experimental methods arises in mental economists are recruiting subjects in large part from the potential for constructing the field rather than in the classroom, using the proper counterfactual. field goods rather than induced valuations, Field experiments provide a meeting ground and using field context rather than abstract between these two broad approaches to empirical economic science. By examining the 2 When we talk about combining lab and field data, we nature of field experiments, we seek to make it do not just mean a summation of conclusions. Instead, we have in mind the two complementing each other in some a common ground between researchers. functional way, much as one might conduct several lab We approach field experiments from the experiments in order to tease apart potential confounds perspective of the sterility of the laboratory For example, James Cox(2004) demonstrates nicely how 'trust" and"reciprocity" are often confounded with "other egarding preferences," and can be better identified sep- Harrison: Department of Economics, College of arately if one undertakes several types of experiments Business Administration, University of Central Floridag with the same population. Similarly. Alvin Roth and List: Department of Agricultural and Resource Economics Michael Malouf(1979) demonstrate how the use of dollar and Department of Economics, University of Maryland payoffs can confound tests of cooperative game theory and NBER. We are grateful to Stephen Burks, Colin with less information of one kind (knowledge of the utili- Camerer, Jeffrey Carpenter, Shelby Gerking, R. Mark ty function of the other player), and more information of Isaac, Alan Krueger, John McMillan, Andreas Ortmann another kind (the ability to make interpersonal compar Charles Plott, David Reiley, E. Elisabet Rutstrom, isons of monetary gain), than is usually assumed in the Nathaniel Wilcox, and the referees for generous comments leading theoretical prediction. 1009