The Logic of SamplingMethods of SamplingSampling of ParticipantsSampling TerminologyNon-Probability SamplingSlide 6Theory & Logic of Probability SamplingThe Normal DistributionCharacteristics of the Normal DistributionThe Standard Deviation and the Normal DistributionProbability SamplingSlide 12Hidden PopulationsSample SizeDrawing InferencesThe Logic of SamplingMethods of SamplingMethods of Sampling •Nonprobability samples Nonprobability samples –Used often in Qualitative ResearchUsed often in Qualitative Research•Probability or random samples Probability or random samples –Every person has an equal chance Every person has an equal chance of being included in the sampleof being included in the sampleSampling of ParticipantsSampling of Participants•Try to obtain a representative sample–Representative samples allow us to generalize findings to the larger group•Sampling is often not under the control of the researcher in low-constraint (field) research–Therefore, caution is required in interpreting the results–Generalize only to similar participants and NOT to the general populationSampling TerminologySampling Terminology•Populations•Sampling Element•Target Population•Sampling Frame•Parameters and StatisticsNon-Probability SamplingNon-Probability Sampling•Convenience or Accidental or Haphazard•Quota•Purposive or Judgmental•SnowballNon-Probability SamplingNon-Probability Sampling•Deviant cases•Sequential•Theoretical•Use of InformantsTheory & Logic of Probability SamplingTheory & Logic of Probability Sampling•Sampling Distribution•Central Limit Theorem•Sampling ErrorThe Normal DistributionThe Normal Distribution•Represents the actual distribution of naturally occurring data•Real distributions do not conform completely to the normal distribution•Inferential statistics takes a set of data and “normalizes” it so comparisons can be madeCharacteristics of the Normal DistributionCharacteristics of the Normal Distribution•Bell shape•Unimodal•Mean is located at the center of the bell curve•Area under the curve is 100% of the data•The 50th percentile or the median, is the same value as the meanThe Standard Deviation and the The Standard Deviation and the Normal DistributionNormal Distribution•Direct relationship between the standard deviation and the curve•The same number of observations will always fall within the same standard deviation units from the mean of the distribution–68% lie within -1 to +1 s.d.’s from the mean–95% lie within -2 to +2 s.d.’s from the mean–99.8% lie within -3 to +3 s.d.’s from the meanProbability SamplingProbability Sampling•Simple Random Sample•Systematic Sampling•Stratified SamplingProbability SamplingProbability Sampling•Cluster Sampling–Within Household Sampling–Probability Proportionate to Size (PPS)•Random-Digit DialingHidden PopulationsHidden Populations•Targeted Sampling•Respondent Drive SamplingSample SizeSample Size•Degree of precision or accuracy needed–Larger samples will provide more precise estimates of population parameters•Variability or diversity in the population •Number of different variables•Costs and time constraints•The larger the sample, the more narrow the confidence intervalsDrawing InferencesDrawing Inferences•Inferential Statistics•Sampling