add another reference

Author: drbenvincent

docs/source/notebooks/did_pymc_banks.ipynb

@@ -833,7 +833,7 @@
     "\n",
     "Finally, we can evaluate the difference in difference model in its two-way fixed effects (TWFE) formulation. The two-way fixed effects model is widely used in econometrics for causal inference in panel data settings. It accounts for both unit-specific effects (e.g., differences between districts) and time-specific effects (e.g., shocks or trends affecting all units simultaneously). \n",
     "\n",
-    "The TWFE model is equivalent to the classic 2$\\times$2 DiD model (Model 1) - but only in the situation of two groups and two time periods. Outside of this special case the approach is not equivalent and can potentially have some problems {cite:p}`imai2021twfepanel`. Readers should proceed with caution when using the TWFE model outside of the 2$\\times$2 case - see {cite:t}`kropko2018two`.\n",
+    "The TWFE model is equivalent to the classic 2$\\times$2 DiD model (Model 1) - but only in the situation of two groups and two time periods. Outside of this special case the approach is not equivalent and can potentially have some problems {cite:p}`imai2021twfepanel`. Readers should proceed with caution when using the TWFE model outside of the 2$\\times$2 case - see for example{cite:p}`kropko2018two,collischon2020let`.\n",
     "\n",
     "The TWFE approach similar to the previous model in that the `district:post_treatment` interaction term still gives you a treatment indicator variable and the assiated coefficient $\\beta_{\\Delta}$ is the causal effect of the intervention. But it is different in that there is no _linear_ `year` term, instead we have a _categorical_ `year` variable. This means that the model can capture any temporal trends in the data. These can be thought of as capturing time based schocks that affect all units.\n",
     "\n",

docs/source/references.bib

@@ -147,3 +147,14 @@ @article{kropko2018two
   pages={e0231349},
   year={2018}
 }
+
+@article{collischon2020let,
+  title={Let's talk about fixed effects: Let's talk about all the good things and the bad things},
+  author={Collischon, Matthias and Eberl, Andreas},
+  journal={KZfSS K{\"o}lner Zeitschrift f{\"u}r Soziologie und Sozialpsychologie},
+  volume={72},
+  number={2},
+  pages={289--299},
+  year={2020},
+  publisher={Springer Fachmedien Wiesbaden}
+}