Bob Erikson writes,

I was trolling the internet and came across your debate with Jens H. from Feb 15 07 on your blog about differences in differences.

You might find the attached document of interest. It is a once-influential currently-obscure article from half a century ago on this topic. The language is not contemporary. But note Campbell’s example of 2 ways to analyze the substantive problem and two very different interpretations. Presumably Campbell is correct, using a difference of differences approach.

Bob’s office is just across from mine in the political science department, but of course we communicate via blogs and emails. Anyway, I’ll have to read the paper carefully. Also it will be interesting to see if they noticed that before-after correlations are higher for controls than for treated units.