In 1978 I had a Guggenheim Fellowship to pursue work on causal inference. I had recently completed my Annals of Statistics article, Rubin (1978a), dealing with Bayesian inference for causal effects, and I was actively consulting with June Reinisch, Director of the Kinsey Institute, on matching problems with a large Danish cohort (used for illustrative purposes in several articles, including Rosenbaum and Rubin (1985a, b), both reprinted in this part, and Rubin and Thomas (1996, 2000), both reprinted in Part V. Some of the empirical context for this problem appears in Chapter 25 in Part VII.

Starting in the winter of 1978, I was able to take a one-semester leave from ETS to visit Harvard University, and taught a seminar on causal inference. There was an outstanding first-year PhD student who took my seminar and ended up writing his thesis largely under my supervision – Paul Rosenbaum. The combination of his brilliance and our real data problems led to a series of papers on propensity score methods that appear to have started an avalanche of very recent contributions addressing propensity score methods; Google, at the end of 2005, lists close to a million entries for “propensity scores” or “propensity score.”

Chapter 10, Rosenbaum and Rubin (1983a), introduced the propensity score. It provided definitions of balancing scores, propensity scores, strongly ignorable treatment assignment (a stronger version of ignorable treatment assignment, defined in Rubin (1978a) but implied in Rubin (1976a)).