Mostly Harmless Econometrics: An Empiricist’s Companion

# The Experimental Ideal

It is an important and popular fact that things are not always what they seem. For instance, on the planet Earth, man had always assumed that he was more intelligent than dolphins because he had achieved so much—the wheel, New York, wars and so on—while all the dolphins had ever done was muck about in the water having a good time. But conversely, the dolphins had always believed that they were far more intelligent than man-for precisely the same reasons. In fact there was only one species on the planet more intelligent than dolphins, and they spent a lot of their time in behavioral research laboratories running round inside wheels and conducting frighteningly elegant and subtle experiments on man. The fact that once again man completely misinterpreted this relationship was entirely according to these creatures’ plans.

Douglas Adams, The Hitchhiker’s Guide to the Galaxy (1979)

The most credible and influential research designs use random assignment. A case in point is the Perry preschool project, a 1962 randomized experiment designed to asses the effects of an early-intervention program involving 123 Black preschoolers in Ypsilanti (Michigan). The Perry treatment group was randomly assigned to an intensive intervention that included preschool education and home visits. It’s hard to exaggerate the impact of the small but well-designed Perry experiment, which generated follow-up data through 1993 on the participants at age 27. Dozens of academic studies cite or use the Perry findings (see, e. g., Barnett, 1992). Most importantly, the Perry project provided the intellectual basis for the massive Head Start pre-school program, begun in 1964, which ultimately served (and continues to serve) millions of American children.[3]

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## Mostly Harmless Econometrics: An Empiricist’s Companion

### Regression Details

1.4.1 Weighting Regression Few things are as confusing to applied researchers as the role of sample weights. Even now, 20 years post - Ph. D., we read the section of …

### Getting a Little Jumpy: Regression Discontinuity Designs

But when you start exercising those rules, all sorts of processes start to happen and you start to find out all sorts of stuff about people... Its just a way …

### Two-Sample IV and Split-Sample IVF

GLS estimates of Г in (4.3.1) are consistent because E The 2SLS minimand can be thought of as GLS applied to equation (4.3.1), after multiplying by /N to keep the …

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