Difference in differences (DID) is a quasi-experimental technique used in econometrics that measures the effect of a treatment at a given period in time. It is often used to measure the change induced by a particular treatment or event, though may be subject to certain biases (mean reversion bias, etc.).

In contrast to a within-subjects estimate of the treatment effect (that measures the difference in an outcome after and before treatment) or a between-subjects estimate of the treatment effect (that measures the difference in an outcome between the treatment and control groups), the DID estimator represents the difference between the pre-post, within-subjects differences of the treatment and control groups.

Thank you Wikipedia for that introduction.  Gamos has recently completed a Difference of Differences study for an NGO that was working with a complex mobilisation of communities around HIV.  Unfortunately its embargoed until the end of May 2012 - so watch this space - more to come.