Contents
What are pre trends?
A common diagnostic approach in such settings is to look at whether the policy change appears to have an effect on the outcome before it actually occurs. 1 The presence of such pre-event trends, or “pre-trends,” is taken as evidence against the strict exogeneity of the policy change.
Who invented difference in differences?
The difference-in-difference (DID) technique originated in the field of econometrics, but the logic underlying the technique has been used as early as the 1850’s by John Snow and is called the ‘controlled before-and-after study’ in some social sciences.
Is the pre-trend common assumption reasonable in a difference in difference framework?
There is a good manner to verify if the pre-trend common assumption is reasonable in a difference-in-difference framework with two time and two periods. But is necessary to have some data for more than one pre-treatment period (Sometimes, the DiD with two periods performs better than the DiD with multiple periods).
What should the pre treatment trends look like?
The typical thing to do is visual inspection of the pre-treatment trends for the control and treatment group. This is particularly easy if you only have those two groups given a single binary treatment. Ideally the pre-treatment trends should look something like this:
Which is the best model for difference in difference estimation?
The linear probability model is the easiest to implement but have limitations for prediction. Logistic models require an additional step in coding to make the interaction terms interpretable. Stata code is provided for this step. Abadie, Alberto. Semiparametric Difference-in-Difference Estimators.
Is it necessary to test for parallel trends?
Testing for Parallel Trends in the Pre-Treatment Period Provided there are enough time points, researchers often test whether trends are parallel in the pre-intervention period. But the test of parallel trends is neither necessary nor sufficient to establish validity of diff-in-diff (Kahn-Lang and Lang 2018 ) .