Although we cannot create a parallel universe in which BarackObama tweeted something else 1 , fortunately, a surprising characteristic of Twitter allows us to run a fairly analogous natural experiment: external forces serendipitously provide an environment that resembles the desired controlled setting (DiNardo, 2008).
it turns out to be unexpectedly common for the same user to post different tweets regarding the same URLa good proxy for fine-grained topic 2 -within a relatively short period of time.