Sampling

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# Sampling - PowerPoint PPT Presentation

Sampling. 11/29/2012. Readings. Chapter 8 Correlation and Linear Regression (Pollock) ( pp 199- 206) Chapter 6 Foundations of Statistical Inference (Pollock) ( pp 122-135). Homework Due Today. Homework Due 11/29 Chapter 8 Question 1: A, B,C,D Question 2: A, B, C, D, E

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## Sampling

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### Sampling

11/29/2012

• Chapter 8 Correlation and Linear Regression (Pollock) (pp 199- 206)
• Chapter 6 Foundations of Statistical Inference (Pollock) (pp 122-135)
Homework Due Today
• Homework Due 11/29
• Chapter 8
• Question 1: A, B,C,D
• Question 2: A, B, C, D, E
• Question 3: A, B, C
• Question 4: A, B, C, D
• Question 5: A, B, C, D, E, G
Final Exam
• SEC 1
• December 12th (Wednesday)
• 1:30 pm - 3:30 pm
• SEC 2
• December 11th (Tuesday)
• 1:30 pm - 3:30 pm
Final Paper
• Due 12/7/2012 by 11:00AM- Doyle 226B
• Turnitin Copy by 11:59PM on 12/7
Office Hours For the Week
• When
• Friday 11-1
• Monday 11-1
• Tuesday 8-12
• And by appointment
Course Learning Objectives
• Students will learn the basics of polling and be able to analyze and explain polling and survey data
• Students will learn the basics of research design and be able to critically analyze the advantages and disadvantages of different types of design.
Question Reliability and Validity
• People Must see value
• Pre-Test
• Pilot Test

### Sampling

After we write the survey, we have to select people!

The Laws of Sampling
• if cost is not a major consideration it is better to collect data for ones target population than for a sample thereof
• if cost dictates that a sample be drawn, a probability sample is usually preferable to a nonprobability sample.
• The Law of Large Numbers
• The accuracy of estimates is expressed in terms of the margin or error and the confidence level.
• all probability samples yield estimates of the target population.
What is a population

A Population

Is the census A population?

The Census includes 225 Million Adults

Cost 11.8 billion dollars

Takes years to compile

Misses out on millions and is obsolete before it is published

• The responses of every unit in your group
• 50 states
• Every student, etc
• There is no “sample” error here
What is Sampling?
• Sampling is the technique of selecting a representative part of a population to estimate the total population
The Sample
• A sample is considerably smaller than the total population.
• Samples that are said to mirror the population are said to be representative.
The Concept of Sampling
• Blood Tests
• Food Tests
Collecting a sample
• Population
• Sampling Frame
• The Sample itself
If cost dictates that a sample be drawn, a probability sample is usually preferable to a nonprobability sample.
Probability vs. Non Probability
• Probability- Every Unit Has a Chance of Being Selected
• Also called a random sample
• Non-Probability- some units have a greater chance of selection
• Usually not generalizable
Why Non-Probability
• Very Fast
• Very cheap
• Difficult Populations to reach
• Exploration
• Get the Product out
• Focus Groups (5:48)
• Alternate endings
• Test audiences
• If it works, you expand
Self Selected Samples
• People Choose to Be in the Sample
• Classic Sanjaya
• Certain people have much more incentive to participate
Straight-up Internet Surveys
• These are self-selected
• Big numbers mean nothing
The Literary Digest in 1936
• Correct in 24,28,32
• 10 million ballots distributed
• 2.2 Million Responses
• Alf Landon Will defeat FDR (by a landslide)
Why The Literary Digest was Wrong
• The wrong sampling frame
• Response bias
• The Literary Digest goes out of business
Convenience Samples
• Super-Fast
• Pick easy targets
Quota/Judgment Samples
• Find People who Match your criteria
• The Price is Right
• Easy, but Not random… not representative
Quota Samples
• A Type of Judgment Sample
• Break the nation into groups
• Pick a certain number/quota from each group
• Stop when you have filled up your quota
The Death of Quota Sampling: 1948
• We used to use these for national polls
• George Gallup thrived on these.
• In 1948 he predicts that Thomas Dewey of New York would defeat Harry Truman
Why Gallup was Wrong
• It was a close election
• The electorate diversified (missed out on groups)
• They filled up quotas with easy targets
• They stopped polling
Snowball Sample
• one becomes two, becomes four, becomes 8
• Difficult to Reach Populations
• Background Checks
Looking through A Parent’s eyes

The Most Beautiful Kids Ever

Internal Polling