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Research Methods Chapter 5: Sampling Sampling Purpose: To draw enough of something to make your findings generalizable Some things to do before conducting a sample Consider a census Evaluate Generalizability Sample generalizability: Can the findings from the sample be generalized to the population from which that sample was taken? Cross-population generalizability: Can the findings from one population be generalized to another slightly different population? Assess the diversity of the sample Shoot for a representative sample… Sampling Methods Non-probability Samples Often used in qualitative research Probability Samples Often used in quantitative research Nonprobability Sampling Why do it? Random sampling may not be possible or is too expensive May be doing exploratory research Types of nonprobability samples Availability sample Quota sample Purposive sample Snowball sample Probability Samples A means that allows us to know in advance the likelihood that an element will be selected from the population Relies on random selection Problems to watch out for when selecting a random sample: An incomplete sampling frame Failure to obtain an adequate response rate Random samples and sampling error Generally, a random sample has sampling error due to chance Use inferential statistics to calculate sampling error 2 things effect the degree of error due to chance: The size of the sample The Homogeneity of the population Probability Sampling Continued Importance: Alf Landon vs. Roosevelt presidential sample Types of probability samples Simple random sample (SRS) Systematic random sample Stratified random sample Cluster sample Sampling Distributions Errors may occur when drawing samples (the sample is not representative of the population) Two reasons why this can occur You make a mistake (systematic sampling error) Errors due to chance (random sampling error) Use inferential statistics to determine sampling error Confidence Intervals Usually reported at 95%, 99%, & 99.9% Determining Sample Size Usually use about 1,000 - 1,500 for U.S. population if looking for a simple description Usually use up to 2,500 if wanting to know about something detailed Locally or regionally, typically a few hundred