▁by ▁T oby ▁Wh ith ouse ▁Category : Do ctor ▁Who ▁pseud oh istor ical ▁serial s ▁Category : Do ctor ▁Who ▁stories ▁set ▁on ▁Earth ▁Category : F iction ▁set ▁in ▁ 1 8 7 0 ▁Category : T ele vision ▁episodes ▁set ▁in ▁the ▁United ▁States <0x0A> </s> ▁In ▁col lo qu ial ▁language , ▁an ▁average ▁is ▁a ▁single ▁number ▁taken ▁as ▁representative ▁of ▁a ▁list ▁of ▁numbers . ▁D ifferent ▁concepts ▁of ▁average ▁are ▁used ▁in ▁different ▁context s . ▁O ften ▁" aver age " ▁refers ▁to ▁the ▁arithmetic ▁mean , ▁the ▁sum ▁of ▁the ▁numbers ▁divided ▁by ▁how ▁many ▁numbers ▁are ▁being ▁aver aged . ▁In ▁statistics , ▁mean , ▁median , ▁and ▁mode ▁are ▁all ▁known ▁as ▁measures ▁of ▁central ▁t endency , ▁and ▁in ▁col lo qu ial ▁usage ▁any ▁of ▁these ▁might ▁be ▁called ▁an ▁average ▁value . ▁ ▁Calcul ation ▁ ▁P yth ag ore an ▁means ▁ ▁The ▁arithmetic ▁mean , ▁the ▁geometric ▁mean ▁and ▁the ▁harm onic ▁mean ▁are ▁known ▁collect ively ▁as ▁the ▁P yth ag ore an ▁means . ▁ ▁Ar ithmetic ▁mean ▁ ▁The ▁most ▁common ▁type ▁of ▁average ▁is ▁the ▁arithmetic ▁mean . ▁If ▁▁ ▁n ▁▁▁▁ ▁numbers ▁are ▁given , ▁each ▁number ▁denoted ▁by ▁ai ▁( where ▁i   =   1 , 2 ,   ...,   n ), ▁the ▁arithmetic ▁mean ▁is ▁the ▁sum ▁of ▁the ▁as ▁divided ▁by ▁n ▁or ▁ ▁The ▁arithmetic ▁mean , ▁often ▁simply ▁called ▁the ▁mean , ▁of ▁two