Instrumental variables

Social Science Statistics Blog: The "Imperial Grip" of Instrumental Variables: "Consider the two papers desribed in the Economist article. The first attempts to estimate the effect of colonialism on current economic outcomes. The authors propose wind speed and direction as an instrument for colonization, arguing (plausibly) that Europeans were more likely to colonize an island if they were more likely to encounter it while sailing. So far so good. Then they argue that, while colonization in the past has an effect on economic outcomes in the present, being situated in a location favorable for sailing in the past (i.e., before steam-powered ships) does not. Is this really plausible? The authors think so, I don't, and it isn't obvious that there is a way to resolve the matter."