Presentation on theme: "Chapter 2 Samples and Populations"— Presentation transcript:
1 Chapter 2 Samples and Populations Sample vs. PopulationDesignMethodsConstructionErrors
2 Population/Target Population Sample vs. PopulationPopulation – the totality of subjects under considerationTarget Population – consists of all subjects considered in the studySample – a portion or a subset of the population for data collection and analysisPopulation/Target PopulationSample
3 Young-adults and older Sample vs. PopulationPopulationTarget PopulationSampleKalamazooYoung-adults and olderHouse-holds
4 Census vs. Sample Survey Census – collection of data using all subjects in the populationSample Survey – collection of data from a representative sample of the populationPopulation/Target PopulationSampleNote: Random Samples should be representative of the population
5 Study Design or Protocol Design Steps involved in solving problems How do I solve this problem? ? ? Study design is done prior to data collection. It involves methods in data collection, analysis of the data and conclusions to be made.
6 Probability vs. Non-Probability Sampling Probability Sampling – subjects are chosen by chanceNon-probability Sampling – can be used for informal and less scientific studiesNote: Non-probability sampling tend to be less representative of the target population
7 Methods in Probability Sampling Simple Random Sampling (SRS) – samples are randomly selected from the populationK-in-1 Systematic Sampling – Every kth subject is chosenStratified Random Sampling – population is divided into subgroups called strata and SRS chosen from each strataCluster sampling – population is divided into subgroups called clusters and clusters are randomly chosen as samples.
8 Example: Household Expenditures in Michigan Target Population : Households in MichiganSimple Random Sampling – randomly selecting the sample from a list of householdsSystematic Sampling – every 10th householdStratified Sampling – take samples from each countyCluster Sampling – selecting counties in Michigan
9 Factors to be considered in a Survey MoneyTimeContent/Information
10 Types of Surveys Type of Survey Advantages Disadvantages Face-to-face InterviewExplain questions, explore issues, make observations, use visual aidsExpensive, need interviewer training- at home orworkAccuracy, better samplingExpensive- in public areasCheaper, more people in less timeLess representative sampleTelephone InterviewAccurate, cheapNo personal observationWritten QuestionnaireCheapest per respondentBias from low response rate- by mailAllows anonymitySlow- byCheaper, quicker results- web surveyQuicker data processingNeed computing expertise
11 Construction of Questionnaire Is the question understandable?Are you gathering knowledge or attitude?Are the questions loaded?Do the questions ask for sensitive information?Note: An accurate answer leads to a good study and it starts from asking important questions correctly.
12 Types of Survey ErrorsCoverage errors – sampling frame excludes some segments of the target populationNon-response errors – can cause bias in survey resultsMeasurement errors – occurs when respondents answer ‘incorrectly’