▁risk ▁directly . ▁However , ▁inc idence ▁can ▁be ▁difficult ▁to ▁measure ▁with ▁r ar er ▁conditions ▁such ▁as ▁aut ism . ▁In ▁aut ism ▁ep ide mi ology , ▁point ▁or ▁period ▁pre val ence ▁is ▁more ▁useful ▁than ▁inc idence , ▁as ▁the ▁condition ▁starts ▁long ▁before ▁it ▁is ▁diagn osed , ▁bearing ▁in ▁mind ▁gen etic ▁elements ▁it ▁is ▁inher ent ▁from ▁conception , ▁and ▁the ▁gap ▁between ▁init iation ▁and ▁diagn osis ▁is ▁influenced ▁by ▁many ▁factors ▁un related ▁to ▁risk . ▁Research ▁focus es ▁mostly ▁on ▁whether ▁point ▁or ▁period ▁pre val ence ▁is ▁increasing ▁with ▁time ; ▁cum ulative ▁inc idence ▁is ▁sometimes ▁used ▁in ▁studies ▁of ▁birth ▁coh ort s . ▁ ▁Est imation ▁methods ▁The ▁three ▁basic ▁approaches ▁used ▁to ▁estimate ▁pre val ence ▁differ ▁in ▁cost ▁and ▁in ▁quality ▁of ▁results . ▁The ▁simplest ▁and ▁che apest ▁method ▁is ▁to ▁count ▁known ▁aut ism ▁cases ▁from ▁sources ▁such ▁as ▁schools ▁and ▁clin ics , ▁and ▁divide ▁by ▁the ▁population . ▁This ▁approach ▁is ▁likely ▁to ▁under est imate ▁pre val ence ▁because ▁it ▁does ▁not ▁count ▁children ▁who ▁have ▁not ▁been ▁diagn osed ▁yet , ▁and ▁it ▁is ▁likely ▁to ▁generate ▁ske wed ▁statistics ▁because ▁some ▁children ▁have ▁better ▁access ▁to ▁treatment . ▁ ▁The ▁second ▁method ▁impro ves ▁on ▁the ▁first ▁by ▁having ▁investig ators ▁examine ▁student ▁or ▁patient ▁records ▁looking ▁for ▁probable ▁cases , ▁to ▁catch ▁cases ▁that ▁have ▁not ▁been ▁identified ▁yet . ▁The ▁third ▁method , ▁which ▁is ▁arg u