Field experiment

A field experiment applies the scientific method to experimentally examine an intervention in the real world (or as many experimental economists like to say, naturally-occurring environments) rather than in the laboratory. Field experiments generally randomize subjects (or other sampling units) into treatment and control groups and compare outcomes between these groups. Clinical trials of pharmaceuticals are one example of field experiments. Economists have used field experiments to analyze discrimination, health care programs, and education programs. Modern social psychologists generally avoid field experiments because of the context dependence of experimental outcomes.

History and Philosophy
Of course, the use of experiments in the lab and the field have a long history in the natural and life sciences. In economics, Peter Bohm, University of Stockholm, was one of the first economsits to take the tools of experimental economic methods and move them to the field in the 1970s. Professor Bohm's work on public good provision over different public goods in which people spent earned income in Sweden was pathbreaking work. Many other examples of experimental economics methods were then used in over the decades in non-market valuation research, including work by Charles Plott, William Schulze, Don Coursey, David Brookshire, Richard Bishop, Thomas Heberlien, Jeffrey Bennett, and Jack Knetsch during the 1970s to the present. By extending experiments beyond the lab, early pioneers provided a bridge between laboratory experiments and naturally occurring data.

Experimental methods in economics evolved in and out of controlled laboratory settings since the 1970s, and even prior to this period in other fields. Social psychology also does have a history of some field experiments, including work by pioneering figures Philip Zimbardo, Kurt Lewin and Stanley Milgram. Milgram's famous "six degrees of separation" study, for example, entailed mailing letters across the country to measure linkages in real-world social networks. The use of field experiments in economics has grown recently with the work of Colin Camerer, John A. List, David Lucking-Reiley, Michael Kremer, Bradley Ruffle, among others.

Applications
Recent work by Glenn W. Harrison (University of Central Florida) and John A. List (University of Chicago) has established a taxonomy of field experiments. See their paper in the December 2004 issue of the Journal of Economic Literature for a complete treatment or List's website for a quicker overview. Their taxonomy usefully partitions field experiments into three categories ranging from those that most closely resemble traditional laboratory experiments to those that are truly "Natural" field experiments in the sense that the subjects involved are unaware of any treatment taking place. (Note that artificial experiments in social psychology often use deception, so that subjects are also unaware of the true treatment).

See the website for many useful applications of the field experiment method ranging from the analysis of public good contributions and charitable giving to market anomalies and analyses of discrimination.

Caveats

 * Fairness of randomization
 * Contamination of the randomization
 * General equilibrium and "scaling-up"
 * Difficulty of replicability (field experiments often require special access of permission)
 * Limits on ability to obtain informed consent of participants