From the paper of Bacon,2018, p.5, I saw a paragraph as below

The simplest way to illustrate how treatment timing works is to consider a balanced panel dataset with ๐‘‡ periods (๐‘ก) and ๐‘ cross-sectional units (๐‘–) that belong to either an untreated group, ๐‘ˆ; an early treatment group, ๐‘˜, which receives a binary treatment at ๐‘ก * ๐‘˜; and a late treatment group, โ„“, which receives the binary treatment at ๐‘ก * โ„“ > ๐‘ก * ๐‘˜

In this paragraph, I am curious if "k" stand for any word having the meaning "early". For example, "l" is late, "U" is untreated,...


1 Answer 1


You can write the author to know for sure, without this one can only speculate.

I don't think the solution is that there is a word for early that starts with a k. Rather people do not like to use $e$, as it has a meaning as Euler's number, while $i,j,k,l$ are pretty standard indexing letters.


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