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Observational Studies

2. Introduction

Observational studies are ubiquitous, and yet, they are not clearly defined. A classic book on the topic explains that observational studies have two characteristics (Cochran 1983).

  1. An objective to study the casual effects of certain agents, procedures, treatments or programs.
  2. The investigator cannot use controlled experimentation, for one reason or another. That is, the investigator cannot impose on a subject, or withhold from a subject, a procedure or treatment whose effects he desires to discover, or cannot assign subjects at random to different procedures.

It is important to appreciate that the intervention is a manipulable alteration in the status quo and, in that important sense, observational studies are akin to experiments.

And as Rosenbaum (2002:1-2) observes, "A study without a treatment is neither an experiment nor an observational study. Most public opinion polls, most forecasting efforts, most studies of fairness and discrimination, and many of other important empirical studies are neither experiments nor observational studies."

An ambiguity occurs in the fact that control over study design may be less important than the quality of the design itself. In some cases, there are "natural experiments" in which natural or social processes assign subjects to treatments so that a very strong research design follows. The data may then be properly analyzed as an experiment, even a randomized experiment. A well-known example is the draft for the Vietnam war (http://www.landscaper.net/draft.htm). Soldiers were drafted by a lottery, which amounts to random assignment to be drafted or not. Researchers could later study the impact of the draft on such things as subsequent earnings (Angrist, 1990).

Cochran, W.G., (1983) Planning & analysis of observational studies. New York: John Wiley and Sons.
Rosenbaum, P.R., (2002) Observational Studies. 2nd edition. New York: Springer. 
Angrist, J. (1990). Lifetime earnings and the Vietnam era draft lottery: Evidence from social security records. The American Economics Review 83(3): 313-336.