-center-group"></a></dt>
<dd><p>a list where the name of each element is the name of the aggregation level, and the element is a formula of
variable names to be group mean centered; for example to group mean center gender and age within the group student:
 <code>list("student"= ~gender+age)</code>, default value of NULL does not perform any group mean centering.</p></dd>


<dt id="arg-center-grand">center_grand<a class="anchor" aria-label="anchor" href="#arg-center-grand"></a></dt>
<dd><p>a formula of varia