s , ▁death s , ▁mar ri ages , ▁imm igration ▁and ▁em igration ▁and ▁a ▁regular ▁census ▁of ▁population ▁provide ▁information ▁that ▁is ▁key ▁to ▁making ▁sound ▁dec isions ▁about ▁national ▁policy . ▁ ▁A ▁useful ▁summary ▁of ▁such ▁data ▁is ▁the ▁population ▁py ram id . ▁It ▁provides ▁data ▁about ▁the ▁sex ▁and ▁age ▁distribution ▁of ▁the ▁population ▁in ▁an ▁accessible ▁graph ical ▁format . ▁ ▁Another ▁summary ▁is ▁called ▁the ▁life ▁table . ▁For ▁a ▁coh ort ▁of ▁persons ▁born ▁in ▁the ▁same ▁year , ▁it ▁traces ▁and ▁projects ▁their ▁life ▁experiences ▁from ▁birth ▁to ▁death . ▁For ▁a ▁given ▁coh ort , ▁the ▁proportion ▁expected ▁to ▁surv ive ▁each ▁year ▁( or ▁de cade ▁in ▁an ▁ab rid ged ▁life ▁table ) ▁is ▁presented ▁in ▁tab ular ▁or ▁graph ical ▁form . ▁ ▁The ▁ratio ▁of ▁males ▁to ▁females ▁by ▁age ▁indicates ▁the ▁consequences ▁of ▁differ ing ▁mort ality ▁rates ▁on ▁the ▁sex es . ▁Thus , ▁while ▁values ▁above ▁one ▁are ▁common ▁for ▁new born s , ▁the ▁ratio ▁dw ind les ▁until ▁it ▁is ▁well ▁below ▁one ▁for ▁the ▁older ▁population . ▁ ▁Collection ▁National ▁population ▁statistics ▁are ▁usually ▁collected ▁by ▁conduct ing ▁a ▁census . ▁However , ▁because ▁these ▁are ▁usually ▁huge ▁log ist ical ▁exer cis es , ▁countries ▁normally ▁conduct ▁cens uses ▁only ▁once ▁every ▁five ▁to ▁ 1 0 ▁years . ▁Even ▁when ▁a ▁census ▁is ▁conducted ▁it ▁may ▁miss ▁counting ▁everyone ▁( known ▁as ▁under count ). ▁Also , ▁some ▁people ▁counted