Citations for estat ptrends and estat granger

[Giuseppe Polito]

Hi,
I'm reading the STATA manual for didregress postestimation.
There the algorithm for the parallel trends test and granger causality test are explained, but there are no references.
What papers can I cite the proves those algorithms works and are useful?

[Joerg Luedicke (StataCorp)]

Giuseppe, have look at this this paper and references mentioned therein:

Wing, C., Simon, K. and Bello-Gomez, R.A., 2018. Designing difference in difference studies: best practices for public health policy research. Annual review of public health, 39.

[Giuseppe Polito]

Thanks!