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1、Chapter2DemandHistory2.1IntroductionForecastsarenecessaryforeachpartineverystockinglocationastheyareusedinthestockingdecisionsonwhentoreplenishthestockandhowmuch.Thefore-castsaretypicallyrevisedeachmonthasthenewmonthlydemandentrybecomesavailable.Theforecastsarebasedonthehistoryflowofthe
2、demandsoverthepastmonths.Themoreaccuratethedemandhistory,thebettertheforecasts.Thetypicalinventorysystemsavesanywherefrom12to36monthsofdemandhistory.Thedatatosaveisthemonthlydemandhistory,denotedas,x(1),…,x(N),wherex(t)isthedemandinmontht,t=1,theoldestmonthinthehistory,andt=N,themostcur
3、rentmonth.Thedemandhistorytosavevariesdependingonthestockkeep-ingfacility.Often,thedatabasehistoryincludesthemonthlydemandsandmonthlylines,whereeachcustomerorderrepresentsalineandthequantityontheorderisthedemand.Attheendofeachmonth,thetotaldemandsandlinesaretalliedforthemonth.Thedatabas
4、emayalsoincludethedemand-to-dateforthecurrentmonthasitisprogressing.Inservicepartsdistributioncenters,thedemandsfromdealersareoftenclassifiedaseitherregularoremergency.Fordistributioncentersthatsupplyretailstores,thedemandsmaybelabeledasnewstockandreplenishstock.Newstockincludestheiniti
5、aldeliverytothestores.Somestockingfacilitiessavetheirdemandsonaweeklybasisratherthanmonthly.Plantsoftenoperatewithfiscal(ratherthancalendar)monthsthataredenotedas445or454or544.Somedistributioncentershavedemandsthatareclassifiedaseitherregularde-mandsandasother-requirements.Otherrequirem
6、entsarefromout-of-the-ordinarycustomers.Insomedistributioncentersandstores,thedemandsmaybepartitionedasregulardemandsandaspromotiondemands.Anothertypeofdemandiscalledadvancedemandandthisoccurswhenadistributioncenterreceivesacustomerorderthatisnottobeshippeduntilsomefuturedate.Whendemand
7、sareshippedincorrect-ly,thecustomerreturnsthestocktothestockinglocationandthesearelabeledasre-turndemands.Sometimesunusualeventsoccurthatcauseaspikeinthedemand,oranerrorhappensinorderentrywherethequantityorpartnumberentersincorrectly,causingtheentryonthedatabasetobesignificantl
