Difference-in-differences (DiD) permits the comparison of differences in outcomes, before and after an intervention, between groups by controlling for bias from unobserved variables that remain fixed over time.

The current study demonstrated the application of the DiD methodology in CER settings to estimate treatment effects in a heterogeneous MS population, where the Test and Control Cohorts varied greatly.

This study has shown that DiD offers a robust comparison of groups, when propensity score matching and other risk-adjustment methods are not suitable. One potential issue is a jump in health outcomes immediately prior to switching the drug.