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BPS - 5th Ed. Chapter 81 Producing Data: Sampling.

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Presentation on theme: "BPS - 5th Ed. Chapter 81 Producing Data: Sampling."— Presentation transcript:

1 BPS - 5th Ed. Chapter 81 Producing Data: Sampling

2 BPS - 5th Ed. Chapter 82 u Researchers often want to answer questions about some large group of individuals (this group is called the population) u Often the researchers cannot measure (or survey) all individuals in the population, so they measure a subset of individuals that is chosen to represent the entire population (this subset is called a sample) u The researchers then use statistical techniques to make conclusions about the population based on the sample Population and Sample

3 BPS - 5th Ed. Chapter 83 Inferences about the Population u Values calculated from samples are used to make conclusions (inferences) about unknown values in the population u Variability –different samples from the same population may yield different results for a particular value of interest –estimates from random samples will be closer to the true values in the population if the samples are larger –how close the estimates will likely be to the true values can be calculated -- this is called the margin of error

4 BPS - 5th Ed. Chapter 84 Sampling Design u A sampling design describes exactly how to choose a sample from the population. u Question: how can we find out the unemployment rate?

5 BPS - 5th Ed. Chapter 85 Bad Sampling Designs u Voluntary response sampling –allowing individuals to choose to be in the sample u Convenience sampling –selecting individuals that are easiest to reach v Both of these techniques are biased –systematically favor certain outcomes

6 BPS - 5th Ed. Chapter 86 Simple Random Sampling u Each individual in the population has the same chance of being chosen for the sample u Each group of individuals (in the population) of the required size (n) has the same chance of being the sample actually selected u Random selection: –“drawing names out of a hat” –table of random digits –computer software

7 BPS - 5th Ed. Chapter 87 Table of Random Digits u Table B on pg. 692 of text –each entry is equally likely to be any of the 10 digits 0 through 9 –entries are independent of each other (knowledge of one entry gives no information about any other entries) –each pair of entries is equally likely to be any of the 100 pairs 00, 01,…, 99 –each triple of entries is equally likely to be any of the 1000 values 000, 001, …, 999

8 BPS - 5th Ed. Chapter 88 Choosing a Simple Random Sample (SRS) STEP 1: Label each individual in the population STEP 2: Use Table B to select labels at random

9 BPS - 5th Ed. Chapter 89 Stratified Random Sample u first divide the population into groups of similar individuals, called strata u second, choose a separate SRS in each stratum u third, combine these SRSs to form the full sample

10 BPS - 5th Ed. Chapter 810 Multistage Sample u several stages of sampling are carried out u useful for large-scale sample surveys u samples at each stage may be SRSs, but are often stratified u stages may involve other random sampling techniques as well

11 BPS - 5th Ed. Chapter 811 Cautions about Sample Surveys u Undercoverage –some individuals or groups in the population are left out of the process of choosing the sample u Nonresponse –individuals chosen for the sample cannot be contacted or refuse to cooperate/respond u Response bias –behavior of respondent or interviewer may lead to inaccurate answers or measurements u Wording of questions –confusing or leading (biased) questions; words with different meanings

12 BPS - 5th Ed. Chapter 812 Nonresponse u The Pew Research Center for the People and the Press imitated a careful random digit dialing survey and published the results: over 5 days, the survey reached 76% of the households in its chosen sample, but “because of busy schedules, skepticism and outright refusals, interviews were completed in just 38% of households that were reached. ” Combining households that could not be contacted with those who did not complete the interview gave a nonresponse rate of 73%.

13 BPS - 5th Ed. Chapter 813 Response Bias u A door-to-door survey is being conducted to determine drug use (past or present) of members of the community. Respondents may give socially acceptable answers (maybe not the truth!) u For this survey on drug use, would it matter if a police officer is conducting the interview? (bias from interviewer)

14 BPS - 5th Ed. Chapter 814 Wording: Deliberate Bias u “If you found a wallet with $20 in it, would you return the money?” u “If you found a wallet with $20 in it, would you do the right thing and return the money?”

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