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STAT 101 (27)
Chapter 8

Chapter 8 .odt

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Simon Fraser University
STAT 101
Qian( Michelle) Zhou

Chapter 8 – Producing Data: Sampling Population Vs Sample • Population – in statistical study is the entire group of individuals about which we want info • Sample – part of population from which we actually collect information. Use sample to draw conclusion about entire population • Sampling design – how to choose a sample from population • Sample Survey ◦ first step: say what population we want to describe ◦ second step: say what we want to measure. Give definitions of our variable ◦ last step: sampling design How to Sample Badly • Easiest (but not the best) design chooses individuals close at hand ◦ ex) going to mall and asking passing people if they are employed ◦ Convenience Sample – sample selected by taking members of population that are easiest to reach ▪ produce unrepresentable data • Bias – systematically favours certain outcomes ◦ Ex) interviews at mall mostly overrepresent middle class people and underrepresent the poor, also unexperienced interviewers will likely to choose those who dress well, friendly • Voluntary response sample – people who choose themselves by responding to a broad appeal. Are biased b/c people w/ strong opinions most likely respond ◦ a poll Simple Random Samples (SRS) • Interviewer makes the choice; personal choices produce bias • Sample chosen by chance rules out both favouritism by sampler and self-selection by respondents ◦ gives everybody a chance • Simple Random Sample – of size n consists of n individuals from population chosen in such a way that every set of n individuals has equal chance to be the sample actually selected ◦ ex) choosing sample size of 4, from a population size of 28 ◦ choosing slip of paper from hat ◦ slow and inconvenient for big populations ▪ use the Simple Random Sample applet, you can randomize by using table of random digits • Table of Random Digits (Table B) – is a long string of digits from 0,1,2,3,4,5,6,7,8,9 w/ these two properties ◦ each entry in table is equally likely to be any of the 10 digits 0 – 9 ◦ entries are independent of each others. Knowledge of one part of table gives no information about any other part. • Two steps in using the table to choose SRS: ◦ Label – give each number of population a numerical label of the same length ◦ Table – Choose and SRS, read from Table B successive groups of digits of the length you used as labels. Your sample contains the individuals whose label you find in the table ▪ read two digits along, and ignore groups that a
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