Bài giảng Business Research Methods - Chapter 14: Sampling

Chapter 14SamplingMcGraw-Hill/IrwinCopyright © 2011 by The McGraw-Hill Companies, Inc. All Rights Reserved. 14-*Learning ObjectivesUnderstand . . .The two premises on which sampling theory is based.The accuracy and precision for measuring sample validity.The five questions that must be answered to develop a sampling plan.14-*Learning ObjectivesUnderstand . . . The two categories of sampling techniques and the variety of sampling techniques within each category.The various sampling techniques and when each is used.14-*Small Samples Can Enlighten“The proof of the pudding is in the eating.By a small sample we may judge of thewhole piece.”Miguel de Cervantes Saavedra author14-*PulsePoint: Research Revelation80The average number of text messages sent per day by American teens.14-*The Nature of SamplingPopulationPopulation ElementCensusSampleSampling frame14-*Why Sample?Greater accuracyAvailability of elementsGreater speedSampling providesLower cost14-*What Is a Sufficiently Large Sample?“In recent Gallup ‘Poll on polls,’ . . . When asked about the scientific sampling foundation on which polls are based . . . most said that a survey of 1,500 – 2,000 respondents—a larger than average sample size for national polls—cannot represent the views of all Americans.”Frank Newport The Gallup Poll editor in chiefThe Gallup Organization14-*When Is a Census Appropriate?NecessaryFeasible14-*What Is a Valid Sample?AccuratePrecise14-*Sampling Design within the Research Process14-*Types of Sampling DesignsElement SelectionProbabilityNonprobabilityUnrestrictedSimple randomConvenienceRestrictedComplex randomPurposiveSystematicJudgmentClusterQuotaStratifiedSnowballDouble14-*Steps in Sampling DesignWhat is the target population?What are the parameters of interest?What is the sampling frame?What is the appropriate sampling method?What size sample is needed?14-*When to Use Larger Sample?Desired precisionNumber of subgroupsConfidence levelPopulation varianceSmall error range14-*Simple RandomAdvantagesEasy to implement with random dialingDisadvantagesRequires list of population elementsTime consumingLarger sample neededProduces larger errorsHigh cost14-*SystematicAdvantagesSimple to designEasier than simple randomEasy to determine sampling distribution of mean or proportionDisadvantagesPeriodicity within population may skew sample and resultsTrends in list may bias resultsModerate cost14-*StratifiedAdvantagesControl of sample size in strataIncreased statistical efficiencyProvides data to represent and analyze subgroupsEnables use of different methods in strataDisadvantagesIncreased error if subgroups are selected at different ratesEspecially expensive if strata on population must be created High cost14-*Cluster AdvantagesProvides an unbiased estimate of population parameters if properly doneEconomically more efficient than simple randomLowest cost per sampleEasy to do without listDisadvantagesOften lower statistical efficiency due to subgroups being homogeneous rather than heterogeneousModerate cost14-*Stratified and Cluster SamplingStratifiedPopulation divided into few subgroupsHomogeneity within subgroupsHeterogeneity between subgroupsChoice of elements from within each subgroupClusterPopulation divided into many subgroupsHeterogeneity within subgroupsHomogeneity between subgroupsRandom choice of subgroups 14-*Area Sampling14-*Double SamplingAdvantagesMay reduce costs if first stage results in enough data to stratify or cluster the populationDisadvantagesIncreased costs if discriminately used14-*Nonprobability SamplesCostFeasibilityTimeNo need to generalizeLimited objectives14-*Nonprobability Sampling MethodsConvenienceJudgmentQuotaSnowball14-*Key TermsArea samplingCensusCluster samplingConvenience samplingDisproportionate stratified samplingDouble samplingJudgment samplingMultiphase samplingNonprobability samplingPopulationPopulation elementPopulation parametersPopulation proportion of incidenceProbability sampling14-*Key TermsProportionate stratified samplingQuota samplingSample statisticsSamplingSampling errorSampling frameSequential samplingSimple random sampleSkip intervalSnowball samplingStratified random samplingSystematic samplingSystematic varianceAppendix 14aDetermining Sample SizeMcGraw-Hill/IrwinCopyright © 2011 by The McGraw-Hill Companies, Inc. All Rights Reserved. 14-*Random Samples14-*Increasing Precision14-*Confidence Levels & the Normal Curve14-*Standard ErrorsStandard Error(Z score)% of AreaApproximate Degree of Confidence1.0068.2768%1.6590.1090%1.9695.0095%3.0099.7399%14-*Central Limit Theorem14-*Estimates of Dining VisitsConfidenceZ score% of AreaInterval Range (visits per month)68%1.0068.279.48-10.5290%1.6590.109.14-10.8695%1.9695.008.98-11.0299%3.0099.738.44-11.5614-*Calculating Sample Size for Questions involving MeansPrecisionConfidence levelSize of interval estimatePopulation DispersionNeed for FPA14-*Metro U Sample Size for MeansStepsInformationDesired confidence level95% (z = 1.96)Size of the interval estimate .5 meals per monthExpected range in population0 to 30 mealsSample mean10Standard deviation4.1Need for finite population adjustmentNoStandard error of the mean.5/1.96 = .255Sample size(4.1)2/ (.255)2 = 25914-*Proxies of the Population DispersionPrevious research on the topicPilot test or pretestRule-of-thumb calculation1/6 of the range14-*Metro U Sample Size for ProportionsStepsInformationDesired confidence level95% (z = 1.96)Size of the interval estimate .10 (10%)Expected range in population0 to 100%Sample proportion with given attribute30%Sample dispersionPq = .30(1-.30) = .21Finite population adjustmentNoStandard error of the proportion.10/1.96 = .051Sample size.21/ (.051)2 = 8114-*Appendix 14a: Key Terms Central limit theoremConfidence intervalConfidence levelInterval estimatePoint estimateProportionAddendum: Keynote CloseUpMcGraw-Hill/IrwinCopyright © 2011 by The McGraw-Hill Companies, Inc. All Rights Reserved. 14-*Keynote Experiment14-*Keynote Experiment (cont.)Determining Sample SizeAppendix 14aMcGraw-Hill/IrwinCopyright © 2011 by The McGraw-Hill Companies, Inc. All Rights Reserved. 14-*Random Samples14-*Confidence Levels14-*Metro U. Dining Club Study