\BeginKnitrBlock{exercise}\iffalse{-91-84-104-105-110-107-105-110-103-32-97-98-111-117-116-32-97-115-115-117-109-112-116-105-111-110-115-93-}\fi{}<div class="exercise"><span class="exercise" id="exr:unnamed-chunk-15"><strong>(\#exr:unnamed-chunk-15)  \iffalse (Thinking about assumptions) \fi{} </strong></span>What are the assumptions we make when using head circumference as a proxy for intelligence? Are they justified?</div>\EndKnitrBlock{exercise}