6 , ▁p .   5 ) ▁ ▁More ▁equations ▁More ▁generally , ▁consider ▁a ▁linear ▁system ▁of ▁M ▁equations , ▁with ▁M   > ▁ 1 . ▁ ▁An ▁equation ▁cannot ▁be ▁identified ▁from ▁the ▁data ▁if ▁less ▁than ▁M   − ▁ 1 ▁variables ▁are ▁ex cluded ▁from ▁that ▁equation . ▁ ▁This ▁is ▁a ▁particular ▁form ▁of ▁the ▁order ▁condition ▁for ▁identification . ▁( The ▁general ▁form ▁of ▁the ▁order ▁condition ▁de als ▁also ▁with ▁restrictions ▁other ▁than ▁exclus ions .) ▁The ▁order ▁condition ▁is ▁necessary ▁but ▁not ▁sufficient ▁for ▁identification . ▁ ▁The ▁rank ▁condition ▁is ▁a ▁necessary ▁and ▁sufficient ▁condition ▁for ▁identification . ▁In ▁the ▁case ▁of ▁only ▁ex clusion ▁restrictions , ▁it ▁must ▁" be ▁possible ▁to ▁form ▁at ▁least ▁one ▁non van ishing ▁determin ant ▁of ▁order ▁M   − ▁ 1 ▁from ▁the ▁columns ▁of ▁A ▁corresponding ▁to ▁the ▁variables ▁ex cluded ▁a ▁pri ori ▁from ▁that ▁equation " ▁( F isher ▁ 1 9 6 6 , ▁p .   4 0 ), ▁where ▁A ▁is ▁the ▁matrix ▁of ▁coefficients ▁of ▁the ▁equations . ▁This ▁is ▁the ▁general ization ▁in ▁matrix ▁algebra ▁of ▁the ▁requirement ▁" while ▁it ▁does ▁enter ▁the ▁other ▁equation " ▁mentioned ▁above ▁( in ▁ ▁the ▁line ▁above ▁the ▁formulas ). ▁ ▁Rel ated ▁use ▁of ▁the ▁term ▁ ▁In ▁engineering ▁language , ▁the ▁term ▁" parameter ▁identification " ▁is ▁used ▁to ▁indicate ▁a ▁more ▁general ▁subject , ▁which ▁is ▁roughly ▁the ▁same ▁as ▁estimation ▁in ▁statistics . ▁ ▁See