The presence of heterogeneity in treatment effects can create problems for researchers employing a narrow experimental pool in their research. In particular it is often questioned whether the results of a particular experiment can be extrapolated outside the specific location of the study. In this article, we use a population-based field experiment in order to test the extent to which treatment effects for impersonal mobilisation techniques (direct mail and telephone) are sensitive to where they are carried out (geography) and the context of the election in which they were conducted. We find that on the whole it does not much matter where an experiment is conducted: the treatment effects are to all intents and purposes geographically uniform. This has important implications for the external validity of get-out-the-vote field studies more generally, especially where single locations are used. However, there is one important exception to this: experiments carried out in high turnout locations at high salience elections may show larger effects than those carried out in low turnout areas. © 2013 Springer Science+Business Media New York.