1、Ineconomicevaluations,projectalternativesarerepresentedbytheircashflowprofilesoverthenyearsorperiodsintheplanninghorizon.在经济评价中，每一个备选项目都用它在以后N年或者规划周期内的现金流量分布形态一一表示。Thus,theinterestperiodsarenormallyassumedtobeinyearst=0,1,2,.,nwitht=0representingthepresenttime.这样，计息周期一般用年来表示（t=0,1,2，n），t=0时表示当前时刻。Le
2、tBt,xbetheannualbenefitattheendofyeartforaninvestmentprojectxwherex=1,2,.refertoprojectsNo.1,No.2,etc.,respectively.我们用Bt,x来表示一个投资项目x在t年末的年收益，x=1,2表示备选项目的标号No.1,No.2。LetCt,xbetheannualcostattheendofyeartforthesameinvestmentprojectx.我们用Ct,x来表示同一个投资项目x在t年末年成本支出。Thenetannualcashflowisdefinedastheannual
3、benefitinexcessoftheannualcost,andisdenotedbyAt,xattheendofyeartforaninvestmentprojectx.Then,fort=0,1,.,n:一个投资项目x在t年末的净现金流量就用年收益减去年成本费用得到，用At,x表示。t=0,1n时，公式为whereAt,xispositive,negativeorzerodependsonthevaluesofBt,xandCt,x,bothofwhicharedefinedaspositivequantities.At,x是正值、负值或者零值取决于Bt,x与Ct,x的值，其中Bt,x
4、与Ct,x均为正值Oncethemanagementhascommittedfundstoaspecificproject,itmustforegootherinvestmentopportunitieswhichmighthavebeenundertakenbyusingthesamefunds.管理者一旦将资金投入到某一特定的投资项目中，就必须放弃这笔资金投入到其他的项目的获利机会。Theopportunitycostreflectsthereturnthatcanbeearnedfromthebestalternativeinvestmentopportunityforegone.机会成
5、本是在所放弃的的投资方案中可从最优的备选方案中获得的收益。Theforegoneopportunitiesmayincludenotonlycapitalprojectsbutalsofinancialinvestmentsorothersociallydesirableprograms.被放弃的投资项目不仅包括资本项目还包括金融投资或者社会公益项目投资。Managementshouldinvestinaproposedprojectonlyifitwillyieldareturnatleastequaltotheminimumattractiverateofreturn(MARR)fromforegoneopportunitiesasenvisionedbytheorganization.只有当一个投资项目能产出的回报至少等于最低收益率时,决策者才会投资该拟建项目（最低收益时的回报即组织设想的所放弃的投资项目的回报）.
