\BeginKnitrBlock{exercise}\iffalse{-91-73-100-101-110-116-105-102-121-105-110-103-32-99-111-110-102-111-117-110-100-105-110-103-32-118-97-114-105-97-98-108-101-115-93-}\fi{}<div class="exercise"><span class="exercise" id="exr:unnamed-chunk-13"><strong>(\#exr:unnamed-chunk-13)  \iffalse (Identifying confounding variables) \fi{} </strong></span>How could you check whether geography is a confounding variable?</div>\EndKnitrBlock{exercise}