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CHAPTER OVERVIEW

CHAPTER OVERVIEW. Populations and Samples Probability Sampling Strategies Nonprobability Sampling Strategies Sampling, Sample Size, and Sampling Error. SAMPLES AND POPULATIONS. Inferential method is based on inferring from a sample to a population

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CHAPTER OVERVIEW

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  1. CHAPTER OVERVIEW • Populations and Samples • Probability Sampling Strategies • Nonprobability Sampling Strategies • Sampling, Sample Size, and Sampling Error

  2. SAMPLES AND POPULATIONS • Inferential method is based on inferring from a sample to a population • Sample—a representative subset of the population • Population—the entire set of participants of interest • Generalizability—the ability to infer population characteristics based on the sample

  3. CHOOSING A REPRESENTATIVE SAMPLE • Probability sampling—the likelihood of any member of the population being selected is known • Nonprobability sampling--the likelihood of any member of the population being selected is unknown

  4. PROBABILITY SAMPLING STRATEGIES • Simple random sampling • Each member of the population has an equal and independent chance of being chosen • The sample should be very representative of the population

  5. CHOOSING A SIMPLE RANDOM SAMPLE • Define the population • List all members of population • Assign numbers to each member of population • Use criterion to select sample

  6. 23157 48559 01837 25993 05545 50430 10537 43508 14871 03650 32404 36223 38976 49751 94051 75853 97312 17618 99755 30870 11742 69183 44339 47512 43361 82859 11016 45623 93806 04338 38268 04491 49540 31181 08429 84187 36768 76233 37948 21569 USING A TABLE OF RANDOM NUMBERS • Select a starting point • The first two digit number is 68 (not used) • The next number, 48, is used • Continue until sample is complete

  7. KEYS TO SUCCESS IN SIMPLE RANDOM SAMPLING • Distribution of numbers in table is random • Members of population are listed randomly • Selection criterion should not be related to factor of interest!!

  8. USING SPSS TO GENERATE RANDOM SAMPLES • Be sure that you’re in a data file • Click Data > Select Cases • Click Random sample of Cases • Click the Sample Button • Define Sample Size • Click Continue • Click OK (in next dialog box)

  9. SYSTEMATIC SAMPLING • Divide the population by the size of the desired sample: e.g., 50/10 = 5 • Select a starting point at random: e.g., 43 = Heather • Select every 5th name from the starting point

  10. STRATIFIED SAMPLING • The goal of sampling is to select a sample that is representative of the population • But suppose— • That people in the population differ systematically along some characteristic? • And this characteristic relates to the factors being studied? • Then stratified sampling is one solution

  11. STRATIFIED SAMPLING • The characteristic(s) of interest are identified (e.g., gender) • The individuals in the population are listed separately according to their classification (e.g., females and males) • The proportional representation of each class is determined (e.g., 40% females & 60% males) • A random sample is selected that reflects the proportions in the population, (e.g., 4 females & 6 males)

  12. STRATIFICATION ON MORE THAN ONE FACTOR

  13. CLUSTER SAMPLING • Instead of randomly selecting individuals • Units (groups) of individuals are identified • A random sample of units is then selected • All individuals in each unit are assigned to one of the treatment conditions • Units must be homogeneous in order to avoid bias

  14. NONPROBABILITY SAMPLING STRATEGIES • Convenience sampling • Captive or easily sampled population • Not random • Weak representativeness • Quota sampling • Proportional stratified sampling is desired but not possible • Participants with the characteristic of interest are non-randomly selected until a set quota is met

  15. SAMPLES, SAMPLE SIZE, AND SAMPLING ERROR • Sampling error = difference between sample and population characteristics • Reducing sampling error is the goal of any sampling technique • As sample size increases, sampling error decreases

  16. HOW BIG IS BIG? • The goal is to select a representative sample— • Larger samples are usually more representative • But larger samples are also more expensive • And larger samples ignore the power of scientific inference

  17. ESTIMATING SAMPLE SIZE • Generally, larger samples are needed when • Variability within each group is great • Differences between groups are smaller • Because • As a group becomes more diverse, more data points are needed to represent the group • As the difference between groups becomes smaller, more participants are needed to reach “critical mass” to detect the difference

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