a></dt>
<dd><p>the number of periods of anticipation (i.e., number
of periods before the treatment happens where the treatment can "already"
affect the outcome)</p></dd>


<dt id="arg-base-period">base_period<a class="anchor" aria-label="anchor" href="#arg-base-period"></a></dt>
<dd><p>The type of base period to use.  This only affects
the numeric value of results in pre-treatment periods.  Results in
post-treatment periods are not affected by this choice.  The default
is "varying", where the base period wi