▁{ 1 , 2 ,..., n }), ▁arranged ▁in ▁such ▁a ▁way ▁that ▁there ▁is ▁an ▁integer ▁t ▁so ▁that ▁for ▁every ▁selection ▁of ▁t ▁columns ▁of ▁the ▁table , ▁all ▁ordered ▁t - tu ples ▁of ▁the ▁symbols , ▁formed ▁by ▁taking ▁the ▁entries ▁in ▁each ▁row ▁restricted ▁to ▁these ▁columns , ▁appear ▁the ▁same ▁number ▁of ▁times . ▁The ▁number ▁t ▁is ▁called ▁the ▁strength ▁of ▁the ▁orthogonal ▁array . ▁Here ▁is ▁a ▁simple ▁example ▁of ▁an ▁orthogonal ▁array ▁with ▁symbol ▁set ▁{ 1 , 2 } ▁and ▁strength ▁ 2 : ▁ ▁{ | ▁class =" wik itable " ▁| - ▁| ▁ 1 ▁|| ▁ 1 ▁|| ▁ 1 ▁| - ▁| ▁ 2 ▁|| ▁ 2 ▁|| ▁ 1 ▁| - ▁| ▁ 1 ▁|| ▁ 2 ▁|| ▁ 2 ▁| - ▁| ▁ 2 ▁|| ▁ 1 ▁|| ▁ 2 ▁| } ▁ ▁Notice ▁that ▁the ▁four ▁ordered ▁pairs ▁( 2 - tu ples ) ▁formed ▁by ▁the ▁rows ▁restricted ▁to ▁the ▁first ▁and ▁third ▁columns , ▁namely ▁( 1 , 1 ), ▁( 2 , 1 ), ▁( 1 , 2 ) ▁and ▁( 2 , 2 ) ▁are ▁all ▁the ▁possible ▁ordered ▁pairs ▁of ▁the ▁two ▁element ▁set ▁and ▁each ▁appears ▁exactly ▁once . ▁The ▁second ▁and ▁third ▁columns ▁would ▁give , ▁( 1 , 1 ), ▁( 2 , 1 ), ▁( 2 , 2 ) ▁and ▁( 1 , 2 ); ▁again , ▁all ▁possible ▁ordered ▁pairs ▁each ▁appearing ▁once . ▁The ▁same