Capture-recapture methods are of general interest because they can be applied to conventional historical sources to address otherwise intractable questions about the size and dynamics of historical populations. When employed to assess alternative explanations for the long-term trajectory of party activism in Britain—based on data drawn from the South Lewisham Labour Party—they suggest that the falling supply of individuals prepared to become politically involved is more important than the changing demand of parties for activists.

Although social scientists often use propensity-score methods to study databases that contain substantial amounts of bias and missing information, these techniques have not been applied in the historical literature. This article uses propensity-score matching to investigate the impact of Moscow training on the selection of leadership cadres within Britain's Communist Party. These matching techniques can enable the quantitative analysis of a range of previously underutilized historical data.