Irrespective of how well designed or maintained, or how skilfully manufacturing equipment is used, the items produced by such equipment will inevitably be subject to some variation, e.g. the length of steel bars cut on a cropping press. Not only variables, but also other attributes will be subject to some variations. These variations might be caused by one or more factors, but we can classify variations into two categories:
Bila kujali jinsi ya pamoja iliyoundwa au kuendelezwa, au jinsi ujuzi utengenezaji wa vifaa ni kutumika, vitu zinazozalishwa na vifaa kama inevitably kuwa chini ya baadhi ya tofauti, kwa mfano, urefu wa chuma baa kata ya mseto vyombo vya habari. Si tu vigezo, lakini pia sifa nyingine itakuwa chini ya baadhi ya tofauti. tofauti hizi inaweza kuwa unasababishwa na sababu moja au zaidi, lakini tunaweza kuainisha tofauti katika makundi mawili: