Apopulationisacollectionofunits.Examplesare:thepopulationofallstudentstakingSTAT231thisterm(theunitisastudent);thepopulationofallpersonsaged18-25livinginOntarioonJanuary1,2020(theunitisapersonaged18-25);andthepopulationofallcarinsurancepoliciesissuedbyaparticularinsurancecompanyintheyear2019(theunitisacarinsurancepolicy).Aprocessisasystembywhichunitsareproduced.StudentstakingSTAT231nowandintothefuturecouldbeconsideredasunitsinaprocess.Carinsuranceclaimsforaparticularinsurancecompanynowandintothefuturecouldalsobeconsideredasunitsinaprocess.Akeyfeatureofprocessesisthattheyusuallyoccurovertimewhereaspopulationsareoftenstatic(definedatonemomentintime).Definition1Avariateisacharacteristicofaunit.Variatescanbeofdifferenttypes.Iftheunitsarepeoplethenvariatessuchastheirheight,weight,age,andtimeuntilrecurrenceofdiseaseaftermedicaltreatmentareallexamplesofcontinuousvariates.Thelifetimeofanelectricalcomponent(theunit)isalsoacontinuousvariate.Supposetheunitofinterestisaproductionrunofsmartphonesmadebyaparticularcompanythenthenumberofdefectivesmartphonesproducedinarunisanexampleofadiscretevariate.Iftheunitsaretreesinaparticularforestthenthenumberofaphidsonatreeisalsoadiscretevariate.Thenumberofdeathsinayearonaparticularsectionofdangeroushighwayisanotherexampleofadiscretevariate.Whatistheunitinthisexample?Variatessuchashaircolour,universityprogramormaritalstatusofaperson(theunit)areexamplesofcategoricalvariatessincethesevariatesdonottakeonnumericalvalues.Anotherexampleofacategoricalvariatewouldbethepresenceorabsenceofadiseaseinaunit.Sometimes,tofacilitatetheanalysisofthedata,wemightredefinethevariateofinteresttobe1ifthediseaseispresentand0ifthediseaseisabsent.Wewouldnowcallthevariateadiscretevariate.Sincethevariateonlytakesonvalues0or1suchavariateisoftenreferredtoasabinaryvariate.Ifacategoricalvariatehasanaturalorderingthenitiscalledanordinalvariate.Forexample,thesizeofaunitisanordinalvariateifthecategoriesforsizeare:large,medium,small.Anotherexampleofanordinalvariatewouldbetheopinionofaperson(theunit)onagivenstatementinapollforwhichthecategoriesmightbe:stronglyagree,agree,neutral,disagree,stronglydisagree.