▁explained ▁by ▁changes ▁in ▁di agnostic ▁criteria , ▁but ▁a ▁ 2 0 0 6 ▁analysis ▁found ▁that ▁special ▁education ▁data ▁poor ly ▁measured ▁pre val ence ▁because ▁so ▁many ▁cases ▁were ▁und i agn osed , ▁and ▁that ▁the ▁ 1 9 9 4 – 2 0 0 3 ▁U . S . ▁increase ▁was ▁associated ▁with ▁decl ines ▁in ▁other ▁di agnostic ▁categories , ▁indicating ▁that ▁di agnostic ▁substitution ▁had ▁occurred . ▁ ▁A ▁ 2 0 0 7 ▁study ▁that ▁mode led ▁aut ism ▁inc idence ▁found ▁that ▁bro aden ed ▁di agnostic ▁criteria , ▁diagn osis ▁at ▁a ▁younger ▁age , ▁and ▁improved ▁efficiency ▁of ▁case ▁as certain ment , ▁can ▁produce ▁an ▁increase ▁in ▁the ▁frequency ▁of ▁aut ism ▁r anging ▁up ▁to ▁ 2 9 - fold ▁depending ▁on ▁the ▁frequency ▁measure , ▁suggesting ▁that ▁method ological ▁factors ▁may ▁explain ▁the ▁observed ▁increases ▁in ▁aut ism ▁over ▁time . ▁A ▁small ▁ 2 0 0 8 ▁study ▁found ▁that ▁a ▁significant ▁number ▁( 4 0 %) ▁of ▁people ▁diagn osed ▁with ▁p rag m atic ▁language ▁imp air ment ▁as ▁children ▁in ▁previous ▁dec ades ▁would ▁now ▁be ▁given ▁a ▁diagn osis ▁as ▁aut ism . ▁A ▁study ▁of ▁all ▁Dan ish ▁children ▁born ▁in ▁ 1 9 9 4 – 9 9 ▁found ▁that ▁children ▁born ▁later ▁were ▁more ▁likely ▁to ▁be ▁diagn osed ▁at ▁a ▁younger ▁age , ▁supporting ▁the ▁argument ▁that ▁apparent ▁increases ▁in ▁aut ism ▁pre val ence ▁were ▁at ▁least ▁partly ▁due