▁Another ▁theory ▁suggests ▁that ▁learning ▁is ▁all - or - none , ▁that ▁is ▁one ▁lear ns ▁the ▁word ▁pair ▁in ▁a ▁single ▁trial ▁and ▁memory ▁performance ▁is ▁due ▁to ▁the ▁average ▁learned ▁pairs , ▁some ▁of ▁which ▁are ▁learned ▁on ▁earlier ▁tri als ▁and ▁some ▁on ▁later ▁tri als . ▁To ▁examine ▁the ▁valid ity ▁of ▁these ▁theories ▁research ers ▁have ▁performed ▁memory ▁experiments . ▁In ▁one ▁experiment ▁published ▁in ▁ 1 9 5 9 , ▁experimental ▁psych ologist ▁Ir vin ▁Rock ▁and ▁colle ague ▁Walter ▁He imer ▁of ▁the ▁University ▁of ▁Illinois ▁had ▁both ▁a ▁control ▁group ▁and ▁an ▁experimental ▁group ▁learn ▁pairs ▁of ▁words . ▁The ▁control ▁group ▁studied ▁word ▁pairs ▁that ▁were ▁repeated ▁until ▁the ▁participants ▁learned ▁all ▁the ▁word ▁pairs . ▁In ▁the ▁experimental ▁group , ▁the ▁learned ▁pairs ▁remained ▁in ▁the ▁list ▁while ▁un lear ned ▁pairs ▁were ▁subst it uted ▁with ▁recom bin ations ▁of ▁previous ▁words . ▁Rock ▁believed ▁that ▁associations ▁between ▁two ▁items ▁would ▁be ▁strength ened ▁if ▁learning ▁were ▁increment al ▁even ▁when ▁pairs ▁are ▁not ▁correctly ▁re called . ▁His ▁hypothesis ▁was ▁that ▁the ▁control ▁group ▁would ▁have ▁a ▁higher ▁correct ▁recall ▁probability ▁than ▁the ▁experimental ▁group . ▁He ▁thought ▁that ▁repet ition ▁would ▁increase ▁the ▁strength ▁of ▁the ▁word ▁pair ▁until ▁the ▁strength ▁reaches ▁a ▁threshold ▁needed ▁to ▁produce ▁an ▁over t ▁response . ▁If ▁learning ▁were ▁all ▁or ▁none , ▁then ▁the ▁control ▁group ▁and ▁the ▁experimental ▁group ▁should ▁learn ▁the ▁word ▁pairs ▁at ▁the ▁same ▁rate . ▁Rock ▁found