chapter  7
29 Pages

Sample characteristics and sampling error

Sampleandpopulationparameters Oneofourmajorobjectiveswhencarryingoutastatisticalanalysisofdata istoobtainastatementofcharacteristicsthatarerelevanttothetotal population,oratleasttothetargetpopulation(seeChapter2),andthe parametersorcharacteristicsthatweseekare,forexample,thepopulation meanandthepopulationstandarddeviation.Moreconcisely,attimes thesemaybecalledthetruemean,etc.,thistermthusimplyingthatitis thevaluewhichwouldbeobtainedfromanalysingthewholebodyofdata concerningthephenomenonunderstudy.Thevaluesthatinpracticeare obtained,however,areusuallybasedononlypartofthebodyofdata,this beingtheresulteitherofadeliberatechoiceorbecausenomoredataare available,i.e.thesevaluesarebasedononlyasampleoftheconditions. Thuswhatisusuallyobtainedisnotthetrueorpopulationmeanbutthe samplemean;thesameappliestothestandarddeviationtoo.Before proceedingtoanyassessmentofthedifferencesbetweendifferentseriesof data,ortoanyfurtherconclusionsbasedonthemeanandthestandard deviation,itisthereforeessentialthatsomethoughtbegiventothe relationshipbetweenthesesamplestatisticsandthetrueparameters.