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1.3 Gathering Data

1.3 Gathering Data. Objectives: By the end of this section, I will be able to… Explain what a random sample is, and why we need one. Identify other sampling methods. Survey Sampling.

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1.3 Gathering Data

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  1. 1.3 Gathering Data Objectives: By the end of this section, I will be able to… • Explain what a random sample is, and why we need one. • Identify other sampling methods.

  2. Survey Sampling Survey sampling, or polling, has become so widespread that hardly a day goes by without the results of some new poll or survey making the headlines. USA TodayPolitics

  3. RANDOM SAMPLING We must use random sampling to ELIMINATE BIAS. In 1936 the Literary Digest had correctly predicted the past 3 presidents. They tried to predict the winner between Republican Alf Landon and Democrat Franklin Roosevelt. They sent out 10 million ballots and received 25% back, about 2.5 million. They predicted Landon would win… BUT

  4. RANDOM SAMPLING Their sample of 10 million was from a list of citizens who OWNED cars and had telephones. In 1930, there were MILLIONS of citizens who had neither. George Gallup ran a poll 1000 times SMALLER and was able to correctly predict FDR because his sample was chosen more randomly. Use your calc to get a random sample of numbers.

  5. BE CAREFUL WHEN CHOOSING A SAMPLE FOR A SURVEY • If you want to know how many students come from divorced homes, can you just interview all the students at Sallies? • If you want to know an average commute time to work, can you just interview your family and friends? Don’t limit your sample! You want it to be random!

  6. SAMPLING METHODS Stratified Sampling Sometimes researchers want to investigate characteristics of a certain subgroup of a population, like Hispanics or males. The “strata” or subgroup is chosen, and THEN the elements are chosen RANDOMLY.

  7. SAMPLING METHODS 2. Systematic Sampling Easy method where complete randomness is not obtainable, so the sample is obtained by selecting every kth element. Roman legion mutinied… pick every 10th member to be executed. Nice huh?

  8. SAMPLING METHODS Cluster Sampling Used when populations are widely scattered. (Wilmington) The population is divided into clusters and then samples are chosen from each cluster. *Location based Compare North Wilmington to Little Italy Area.

  9. SAMPLING METHODS 4. Convenience Sampling What I use all the time! Subjects are chosen based on what is convenient for the survey personnel. I will survey you many times throughout the semester.

  10. Page 28 Try This in Class…

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