By selection bias and non response bias quality of a

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Unformatted text preview: s √ Quality of a sample affects the estimation of parameters √ To find out if a sample is any good… Just look at how it was chosen. Any selection bias? √ When there is a high non-response rate, look out for non-response bias. NUS/FOS/DSAP 7 GEM2900 Semester 1, 2009/1010 More on Bias in Sampling… Example: The Year the Polls Elected Dewey... Before the US presidential election in 1948, 3 major polls covered election campaign, Crossley, Gallup and Roper, had declared Thomas Dewey a winner over Harry Truman with a lead of 5 percentage points. The result of the election: Dewey 45%; Truman 50%... Truman won! All 3 major polls made the wrong prediction. What went wrong? NUS/FOS/DSAP 8 GEM2900 Semester 1, 2009/1010 More on Bias in Sampling… Example: The Year the Polls Elected Dewey... ◙ Find out how the sample was chosen… The method used to chose sample is called quota sampling. … A fixed number of individuals (quota) with certain key characteristics were pre-determined to resemble the population. … Within assigned quota, the interviewers of the poll were free to choose anyone they like. … Any human choice is, more often than not, subject to bias. NUS/FOS/DSAP 9 GEM2900 Semester 1, 2009/1010 Using Chance in Survey To minimize the bias in sampling, an objective and impartial probability method should be used to draw samples. Some probability methods in sampling… … Simple random sampling √ Each observation in the population has an equal probability of being included in the sample √ It makes statistical sense, but hard to execute in reality… Not practical … Cluster sampling … Stratified sampling NUS/FOS/DSAP 10 GEM2900 Semester 1, 2009/1010 Using Chance in Survey Cluster Sampling: √ Divide population into several subgroups (clusters) √ Each cluster is representative of the population √ Randomly select 1 or few clusters √ Include all observations from selected clusters or simple random samples from selected clusters to form a sample Population divided into few clusters NUS/FOS/DSAP Randomly selected one or few clusters 11 GEM2900 Semester 1, 2009/1010 Using Chance in Survey Stratified Sampling: Strata √ Divide observations to mutually exclusive subgroups √ Observations within the same subgroup have some common properties √ Observations from different subgroups are different with respect to certain properties √ Use simple random sampling to choose observations from each subgroup to form sample NUS/FOS/DSAP 12...
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