Sampling

12/4/2012

Readings• Chapter 8 Correlation and Linear Regression (Pollock)

(pp 199- 206)

• Chapter 6 Foundations of Statistical Inference (Pollock) (pp 122-135)

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

OPPORTUNITIES TO DISCUSS COURSE CONTENT

Office Hours For the Week

• When• Wednesday 8-4• Thursday 10-12

• 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.

Sampling

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

NON-PROBABILITY SAMPLESWhy?

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

The Laws of Sampling

1. if cost is not a major consideration it is better to collect data for ones target population than for a sample thereof

2. if cost dictates that a sample be drawn, a probability sample is usually preferable to a nonprobability sample.

3. The Law of Large Numbers4. The accuracy of estimates is expressed in terms of the

margin or error and the confidence level. 5. all probability samples yield estimates of the target

population.

PROBABILITY SAMPLING

Rules on Sampling

• if cost dictates that a sample be drawn, a probability sample is usually preferable to a nonprobability sample.

• The Law of Large Numbers

Collecting a sample

• Population

• Sampling Frame

• The Sample itself

Probability Samples

• Ensure that every unit in the population has an equal chance of being selected

• In a simple random sample all elements in the population can be selected (SRS)– This involves having a full list of everyone!– We cannot do a SRS of the United States

The best that we can hope for is that every unit in the sampling frame has an equal chance of being selected

How to do it- Simple Way

Random Number Table The Lottery Method

The Law of Large Numbers

• Smaller samples cause greater error.

• The larger the sample size, the greater the probability that our sample will represent the population.

All probability samples yield estimates of the target population

Two Things that Deal With the Stars

Astronomy Astrology

Polling is Science (Astronomy)

• Polls are right more than they are wrong

• We especially love them when it favors our candidates.

Polling is Random (Astrology)

• It is not an exact science, there is error in every poll.

• Polls Don’t Vote, People Vote

• We like it less when it doesn’t favor our candidate

Same Election, Different Results

Poll Date Sample MoE Obama (D)

Romney (R) Spread

Rasmussen Tracking

10/4 - 10/6 1500 LV 3 47 49 Romney +2

Gallup Tracking

9/30 - 10/6 3050 RV 2 49 46 Obama +3

CNN/Opinion Research

9/28 - 9/30 783 LV 3.5 50 47 Obama +3

National Journal

9/27 - 9/30 789 LV 4.2 47 47 Tie

NBC News/WSJ

9/26 - 9/30 832 LV 3.4 49 46 Obama +3

NPR9/26 - 9/30 800 LV 4 51 44 Obama +7

ABC News/Wash Post

9/26 - 9/29 813 LV 4 49 47 Obama +2

Different Questions Perhaps?

• If the election were held today, would you vote for Barack Obama or Mitt Romney?

• If the election were held today, would you vote for Mitt Romney or Barack Obama?

• If the election were held today, would you vote for Democrat Barack Obama or Republican Mitt Romney?

• If the election were held today, would you vote for Republican Mitt Romney or Democrat Barack Obama?

• If the election were held today, for whom would you vote?

More Likely a different sample

SAMPLING ERRORPolling is 95% Science and 5% Astrology

The accuracy of estimates is expressed in terms of the margin or error and the confidence level

The Confidence Level

• The Confidence Level- can we trust these results?

• Surveys use a 95% confidence interval that the results will fall within the margin of error

• There is a 5% (1 out of 20) chance that the results will fall outside this range and produce wacky findings.

• This error often appears when you keep asking the same questions again and again

The Margin of Error

• Margin of Error

• A floating range above and below the estimate.

• Large Samples= Less Error

What else determines sampling error

• Non-response rate

• Variability

• Bias

How Can a Survey of 1000 People Represent Millions of Voters?

• Responses Cancel each other out

• No New opinions are added

Its Logarithmic