Research Methods Chapter 5: Sampling. Sampling Purpose: To draw enough of something to make your findings generalizable Purpose: To draw enough of something.

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Research Methods Chapter 5: Sampling

Sampling Purpose: To draw enough of something to make your findings generalizable Purpose: To draw enough of something to make your findings generalizable Some things to do before conducting a sample Some things to do before conducting a sample Consider a census Consider a census Evaluate Generalizability Evaluate Generalizability Sample generalizability: Can the findings from the sample be generalized to the population from which that sample was taken? 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? Cross-population generalizability: Can the findings from one population be generalized to another slightly different population? Assess the diversity of the sample Assess the diversity of the sample Shoot for a representative sample… Shoot for a representative sample…

Sampling Methods Non-probability Samples Non-probability Samples Often used in qualitative research Often used in qualitative research Probability Samples Probability Samples Often used in quantitative research Often used in quantitative research

Nonprobability Sampling Why do it? Why do it? Random sampling may not be possible or is too expensive Random sampling may not be possible or is too expensive May be doing exploratory research May be doing exploratory research Types of nonprobability samples Types of nonprobability samples Availability sample Availability sample Quota sample Quota sample Purposive sample Purposive sample Snowball 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 A means that allows us to know in advance the likelihood that an element will be selected from the population Relies on random selection Relies on random selection Problems to watch out for when selecting a random sample: Problems to watch out for when selecting a random sample: An incomplete sampling frame An incomplete sampling frame Failure to obtain an adequate response rate Failure to obtain an adequate response rate Random samples and sampling error Random samples and sampling error Generally, a random sample has sampling error due to chance Generally, a random sample has sampling error due to chance Use inferential statistics to calculate sampling error Use inferential statistics to calculate sampling error 2 things effect the degree of error due to chance: 2 things effect the degree of error due to chance: The size of the sample The size of the sample The Homogeneity of the population The Homogeneity of the population

Probability Sampling Continued Importance: Alf Landon vs. Roosevelt presidential sample Importance: Alf Landon vs. Roosevelt presidential sample Types of probability samples Types of probability samples Simple random sample (SRS) Simple random sample (SRS) Systematic random sample Systematic random sample Stratified random sample Stratified random sample Cluster sample Cluster sample

Sampling Distributions Errors may occur when drawing samples (the sample is not representative of the population) Errors may occur when drawing samples (the sample is not representative of the population) Two reasons why this can occur Two reasons why this can occur You make a mistake (systematic sampling error) You make a mistake (systematic sampling error) Errors due to chance (random sampling error) Errors due to chance (random sampling error) Use inferential statistics to determine sampling error Use inferential statistics to determine sampling error Confidence Intervals Confidence Intervals Usually reported at 95%, 99%, & 99.9% Usually reported at 95%, 99%, & 99.9%

Determining Sample Size Usually use about 1, ,500 for U.S. population if looking for a simple description Usually use about 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 Usually use up to 2,500 if wanting to know about something detailed Locally or regionally, typically a few hundred Locally or regionally, typically a few hundred