▁real ▁effects ▁of ▁a ▁program , ▁and ▁the ▁program ▁will ▁' app ear ▁to ▁be ▁less ▁effective ▁than ▁it ▁actually ▁is ' ▁( R os si ▁et ▁al ., ▁ 2 0 0 4 , ▁p .   2 1 9 ). ▁Hence , ▁it ▁is ▁important ▁to ▁ensure ▁the ▁evaluation ▁is ▁as ▁reliable ▁as ▁possible . ▁ ▁Valid ity ▁The ▁valid ity ▁of ▁a ▁measurement ▁instrument ▁is ▁' the ▁extent ▁to ▁which ▁it ▁measures ▁what ▁it ▁is ▁intended ▁to ▁measure ' ▁( R os si ▁et ▁al ., ▁ 2 0 0 4 , ▁p .   2 1 9 ). ▁This ▁concept ▁can ▁be ▁difficult ▁to ▁accur ately ▁measure : ▁in ▁general ▁use ▁in ▁evalu ations , ▁an ▁instrument ▁may ▁be ▁de emed ▁valid ▁if ▁accepted ▁as ▁valid ▁by ▁the ▁st ake hold ers ▁( st ake hold ers ▁may ▁include , ▁for ▁example , ▁fund ers , ▁program ▁administr ators , ▁et ▁c eter a ). ▁ ▁S ens itivity ▁The ▁principal ▁purpose ▁of ▁the ▁evaluation ▁process ▁is ▁to ▁measure ▁whether ▁the ▁program ▁has ▁an ▁effect ▁on ▁the ▁social ▁problem ▁it ▁see ks ▁to ▁red ress ; ▁hence , ▁the ▁measurement ▁instrument ▁must ▁be ▁sensitive ▁enough ▁to ▁disc ern ▁these ▁potential ▁changes ▁( R os si ▁et ▁al ., ▁ 2 0 0 4 ). ▁A ▁measurement ▁instrument ▁may ▁be ▁ins ens itive ▁if ▁it ▁contains ▁items ▁meas uring ▁out comes ▁which ▁the ▁program ▁couldn ' t ▁possibly ▁effect , ▁or ▁if ▁the ▁instrument ▁was ▁originally ▁developed ▁for ▁applications ▁to