▁the ▁real ▁cause ▁of ▁the ▁observed ▁outcome ▁( or ▁the ▁real ▁prevent ion ▁of ▁the ▁anticip ated ▁outcome ). ▁ ▁C aus ation ▁is ▁difficult ▁to ▁determine . ▁One ▁main ▁reason ▁for ▁this ▁is ▁self ▁selection ▁bias . ▁People ▁select ▁themselves ▁to ▁particip ate ▁in ▁a ▁program . ▁For ▁example , ▁in ▁a ▁job ▁training ▁program , ▁some ▁people ▁decide ▁to ▁particip ate ▁and ▁others ▁do ▁not . ▁Those ▁who ▁do ▁particip ate ▁may ▁differ ▁from ▁those ▁who ▁do ▁not ▁in ▁important ▁ways . ▁They ▁may ▁be ▁more ▁determined ▁to ▁find ▁a ▁job ▁or ▁have ▁better ▁support ▁resources . ▁These ▁characteristics ▁may ▁actually ▁be ▁causing ▁the ▁observed ▁outcome ▁of ▁increased ▁employ ment , ▁not ▁the ▁job ▁training ▁program . ▁ ▁E valu ations ▁conducted ▁with ▁random ▁assignment ▁are ▁able ▁to ▁make ▁stronger ▁infer ences ▁about ▁caus ation . ▁Random ly ▁assigning ▁people ▁to ▁particip ate ▁or ▁to ▁not ▁particip ate ▁in ▁the ▁program , ▁reduces ▁or ▁elimin ates ▁self - selection ▁bias . ▁Thus , ▁the ▁group ▁of ▁people ▁who ▁particip ate ▁would ▁likely ▁be ▁more ▁compar able ▁to ▁the ▁group ▁who ▁did ▁not ▁particip ate . ▁ ▁However , ▁since ▁most ▁programs ▁cannot ▁use ▁random ▁assignment , ▁caus ation ▁cannot ▁be ▁determined . ▁Imp act ▁analysis ▁can ▁still ▁provide ▁useful ▁information . ▁For ▁example , ▁the ▁out comes ▁of ▁the ▁program ▁can ▁be ▁described . ▁Thus ▁the ▁evaluation ▁can ▁describe ▁that ▁people ▁who ▁participated ▁in ▁the ▁program ▁were ▁more ▁likely ▁to ▁experience ▁a ▁given ▁outcome ▁than ▁people ▁who ▁did ▁not ▁particip ate