Approximations　and　treatments　commonly　applied　in　multi-group　library　and　self-shielding　calculation　of　conventional　neutronics　calculation　codes　are　analyzed　based　on　a　numerical　nuclear　reactor　physics　code　NECP-X.　The　numerical　results　show　that　the　approximations　and　treatments　introducing　largest　errors　for　UO2　pin　cell　problems　are　narrow　resonance　(NR)　approximation,　ignorance　of　multi-group　(MG)　equivalence　effect,　Bondarenko　iteration　method　and　using　average　fission　spectra.　For　MOX　pin　cell　problems,　the　approximations　and　treatments　introducing　largest　errors　are　ignorance　of　thermal　resonance,　NR　approximation,　resonance　interference　factor　method,　ignorance　of　MG　equivalence　effect　and　in-flow　transport　correction.　These　analysis　provide　references　and　help　to　improve　the　conventional　methods　and　legacy　codes.　(C)　2019　Elsevier　Ltd.　All　rights　reserved.