Event study
REGRESSION, EVENT
Our analysis thus far compares treatment and control counties before and after the campaign began, pooling together all pre-campaign dates and similarly pooling together all post-campaign dates. This pooling does not allow us to see when precisely the vaccine uptake occurred. To examine this, we adopt an event study design that expands on our main regression analysis to estimate ITT effects at each specific date before and after the campaign.The results are shown in The results suggest that the difference between treatment and control counties is not significantly different from zero before the campaign, which offers additional reassurance that our randomization worked as intended. This is a test of the parallel trends assumption required for identification in a difference-in-difference model, evidence that the number of vaccines in treatment counties—and the trend in that number—is not statistically significantly different from that of treatment counties before the start of the campaign. There are several possible explanations for the pattern of point estimates (the solid black lines) we observe in 