st: -rd- on SSC, and a working paper

["Austin Nichols" <austinnichols@gmail.com>]

Thanks to Kit Baum, the new package -rd- is available from SSC;
type -ssc install rd- to get it.

      rd implements a set of regression-discontinuity estimation
      methods that are thought to have very good internal validity, for
      estimating the causal effect of one explanatory variable in the
      case where there is an observable jump (discontinuity) in the
      level of the explanatory variable. Local linear regression using
      -lpoly- or -locpoly- is used to construct point estimates, and
      standard errors are available via -bootstrap-. Graphics dictate
      Stata 8.2 or later.

If using weighted data, I recommend working in Stata 10, since
-locpoly- does not support weights.

A working paper describing regression discontinuity designs and
other methods for "Causal Inference With Observational Data" is
available at http://pped.org/stata/ciwod.pdf
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