I am trying to find causal effects of a policy change in one county in Spain on the absence of certain types of cancer in 2008. My idea was to compare pre- and post-treatment prevalence rates in this county with the national rates (maybe also compare to the prevalence rates of peer counties). (treated vs non-treated ofcourse)
My data are aggregated, meaning I have cancer rates per year from 2000 until 2014 for each and every county (89 in total), see http://imgur.com/XKAxGGD with only 3 counties as an example.
If I plot my data over time ("county1" vs "mean rates of every other county"), the common trends assumption seems to be met, and you can also see the effect of the treatment pretty good.
My question is the following: since my data are not on an individual basis, is it even appropriate/possible to use the regression formulation of diff-in-diff? I don´t see how it would be possible to get sstandard errors off that.
Thank you so much!