# Lecture080 - STAT 350 LECTURE 8 Sampling POPULATION VS...

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STAT 350 L ECTURE 8 Sampling

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P OPULATION VS SAMPLE Example: Presidential Election Population all eligible voters Sample a subset of all eligible voters Statistical Inference Generalizations from the sample to the population Impractical to study the whole population Example of (Population) Parameters: The average age of all eligible voters The percentage of all eligible voters who are currently registered to vote
S TATISTICS Statistics: Numbers which can be computed from a sample Example: A sample of 10,000 Americans We can compute two statistics: The average age of all eligible voters in the sample The percentage of all eligible voters in the sample who are currently registered to vote Statistics are what we know Parameters are what we want to know

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T HE L ITERARY D IGEST P OLL The 1936 Presidential Election Franklin D. Roosevelt vs Alfred Landon The Literary Digest Poll Predicted an overwhelming victory for Landon Predicted Roosevelt getting 43% of popular vote Based on 2.4 million individuals (largest ever) Digest had called the winner in every presidential election since 1916 But … Roosevelt won the 1936 election by a landslide 62% to 38%
G ALLUP S POLL Sample of 50,000 people Correctly forecasted Roosevelt victory Summery of 1936 Election Roosevelt’s Percentage The election result 62 Digest’s Prediction 43 Gallup’s Prediction 56

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S ELECTION B IAS Digest’s Procedure: Mail questionnaires to 10 million people These names and addresses were from telephone books and club membership Back in 1936, ¼ household had a telephone Strong selection bias against the poor When a selection procedure is biased, taking a large sample does not help.
N ON - RESPONSE BIAS Non-response bias -- A large number of those selected for the sample do not respond to the questionnaire or the interview.

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## This note was uploaded on 02/06/2012 for the course STAT 350 taught by Professor Staff during the Spring '08 term at Purdue University-West Lafayette.

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Lecture080 - STAT 350 LECTURE 8 Sampling POPULATION VS...

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