"Faced with the challenge of estimating the elasticity of labor demand in a supply and demand market, the method of Friedman and Schwartz (1963) would be to look for two periods that are adjacent in time, with conditions that were very similar, except for a change that shifts the labor supply curve in one period relative to the other. To find this pair, they would look carefully at historical evidence that they would add to the information in the scatter plot.
If the historical circumstances offer up just one such pair, they would ignore all the other data points and base an estimate on just that pair. If Lucas and Sargent (1979) are correct that the identification problem is the most important problem in empirical macroeconomics, it makes sense to throw away data. It is better to have a meaningful estimate with a larger standard error than a meaningless estimate with a small standard error."
Romer has long recognized the importance of economic history for understanding growth patterns. Here is one of his other well known papers.