. ▁ ▁If ▁the ▁program ▁is ▁fairly ▁large , ▁and ▁there ▁are ▁enough ▁data , ▁statistical ▁analysis ▁can ▁be ▁used ▁to ▁make ▁a ▁reasonable ▁case ▁for ▁the ▁program ▁by ▁showing , ▁for ▁example , ▁that ▁other ▁causes ▁are ▁unlikely . ▁ ▁R eli ability , ▁valid ity ▁and ▁sens itivity ▁in ▁program ▁evaluation ▁It ▁is ▁important ▁to ▁ensure ▁that ▁the ▁instruments ▁( for ▁example , ▁tests , ▁question na ires , ▁etc .) ▁used ▁in ▁program ▁evaluation ▁are ▁as ▁reliable , ▁valid ▁and ▁sensitive ▁as ▁possible . ▁According ▁to ▁Ros si ▁et ▁al . ▁( 2 0 0 4 , ▁p .   2 2 2 ), ▁' a ▁measure ▁that ▁is ▁poor ly ▁chosen ▁or ▁poor ly ▁conce ived ▁can ▁completely ▁und erm ine ▁the ▁worth ▁of ▁an ▁impact ▁assess ment ▁by ▁producing ▁mis leading ▁estimates . ▁Only ▁if ▁outcome ▁measures ▁are ▁valid , ▁reliable ▁and ▁appropri ately ▁sensitive ▁can ▁impact ▁assess ments ▁be ▁regarded ▁as ▁cred ible '. ▁ ▁R eli ability ▁The ▁reli ability ▁of ▁a ▁measurement ▁instrument ▁is ▁the ▁' ext ent ▁to ▁which ▁the ▁measure ▁produces ▁the ▁same ▁results ▁when ▁used ▁repeatedly ▁to ▁measure ▁the ▁same ▁thing ' ▁( R os si ▁et ▁al ., ▁ 2 0 0 4 , ▁p .   2 1 8 ). ▁The ▁more ▁reliable ▁a ▁measure ▁is , ▁the ▁greater ▁its ▁statistical ▁power ▁and ▁the ▁more ▁cred ible ▁its ▁find ings . ▁If ▁a ▁meas uring ▁instrument ▁is ▁un re li able , ▁it ▁may ▁dil ute ▁and ▁obsc ure ▁the