ll "back up" to the immediately
preceding period in pre-treatment periods.  The other option is "universal"
where the base period is fixed in pre-treatment periods to be the period
right before the treatment starts.  "Universal" is commonly used in
difference-in-differences applications, but can be unnatural for other
identification strategies.</p></dd>


<dt id="arg--">...<a class="anchor" aria-label="anchor" href="#arg--"></a></dt>
<dd><p>extra arguments to get the subset correct</p></dd>

</dl></div>
   