<p>You can write the author to know for sure, without this one can only speculate.</p>
<p>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 <span class="math-container">$e$</span>, as it has a meaning as <a href="https://en.wikipedia.org/wiki/E_(mathematical_constant)" rel="nofollow noreferrer">Euler's number</a>, while <span class="math-container">$i,j,k,l$</span> are pretty standard indexing letters.</p>

CC BY-SA 4.0
2021-06-24T07:57:15.183Z
2021-06-24T07:57:15.183Z
Giskard
<p>From the paper of <a href="https://www.nber.org/system/files/working_papers/w25018/w25018.pdf" rel="nofollow noreferrer">Bacon,2018, p.5</a>, I saw a paragraph as below</p>
<blockquote>
<p>The simplest way to illustrate how treatment timing works is to
consider a balanced panel dataset with  periods () and 
cross-sectional units () that belong to either an untreated group,
; an early treatment group, , which receives a binary treatment at
 * ; and a late treatment group, ℓ, which receives the binary
treatment at  * ℓ &gt;  * </p>
</blockquote>
<p>In this paragraph, I am curious if &quot;<em>k</em>&quot; stand for any word having the meaning &quot;early&quot;. For example, &quot;l&quot; is late, &quot;U&quot; is untreated,...</p>

<applied-econometrics>
Is there any special reason to use the character "k" standing for the "early"?
Phil Nguyen
# Is there any special reason to use the character "k" standing for the "early"?

(Asked by: username_0 on 2021-06-24)

From the paper of [Bacon,2018, p.5](https://www.nber.org/system/files/working_papers/w25018/w25018.pdf), I saw a paragraph as below

> The simplest way to illustrate how treatment timing works is to
> consider a balanced panel dataset with periods () and
> cross-sectional units () that belong to either an untreated group,
> ; an early treatment group, , which receives a binary treatment at
> \* ; and a late treatment group, ℓ, which receives the binary
> treatment at \* ℓ > \*

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

**Tags:** applied-econometrics

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## 1 answers

### Answer by username_1 on 2021-06-24. Score: 2

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](https://en.wikipedia.org/wiki/E_(mathematical_constant)), while $i,j,k,l$ are pretty standard indexing letters.


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