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Collecting Data: Sampling

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STAT 101

Dr. Kari Lock Morgan

8/30/12

Collecting Data: Sampling

- SECTION 1.2
- Sample versus Population
- Statistical Inference
- Sampling Bias
- Simple Random Sample
- Other Sources of Bias

A population includes all individuals or objects of interest.

A sampleis all the cases that we have collected data on (a subset of the population).

Statistical inferenceis the process of using data from a sample to gain information about the population.

Sample

The Big Picture

Population

Sampling

Statistical Inference

Which of the following is most important to you?

- Athletics
- Academics
- Social Life
- Community Service
- Other

- Suppose researchers studying student life at Duke use the results of our clicker question to investigate what Duke students find important
- What is the sample?
- What is the population?
- Can the sample data be generalized to make inferences about the population? Why or why not?

Sampling bias occurs when the method of selecting a sample causes the sample to differ from the population in some relevant way.

- If sampling bias exists, we cannot trust generalizations from the sample to the population

- Sampling bias occurs when the method of selecting a sample causes the sample to differ from the population in some relevant way
- If sampling bias exists, we cannot trust generalizations from the sample to the population

Sample

Sampling

Population

Sample

GOAL: Select a sample that is similar to the population, only smaller

- The next slide shows Lincoln’s Gettysburg Address. The entire population, all words in his address, will be shown to you. What is the average word length?
- Your task: Select a sample of 10 words that resemble the overall address. Write them down.
- Calculate the average number of letters for the words in your sample
- Place a dot above your sample average on the board

“Four score and seven years ago our fathers brought forth, on this continent, a new nation, conceived in Liberty, and dedicated to the proposition that all men are created equal. Now we are engaged in a great civil war, testing whether that nation, or any nation so conceived and so dedicated, can long endure. We are met on a great battle-field of that war. We have come to dedicate a portion of that field, as a final resting place for those who here gave their lives that that nation might live. It is altogether fitting and proper that we should do this. But, in a larger sense, we can not dedicate—we can not consecrate—we can not hallow—this ground. The brave men, living and dead, who struggled here, have consecrated it, far above our poor power to add or detract. The world will little note, nor long remember what we say here, but it can never forget what they did here. It is for us the living, rather, to be dedicated here to the unfinished work which they who fought here have thus far so nobly advanced. It is rather for us to be here dedicated to the great task remaining before us—that from these honored dead we take increased devotion to that cause for which they here gave the last full measure of devotion—that we here highly resolve that these dead shall not have died in vain—that this nation, under God, shall have a new birth of freedom—and that government of the people, by the people, for the people, shall not perish from the earth.”

- Actual average: 4.29 letters
- People are TERRIBLE at selecting a good sample, even when explicitly trying to avoid sampling bias!
- We need a better way…

- How can we make sure to avoid sampling bias?
- Imagine putting the names of all the units of the population into a hat, and drawing out names at random to be in the sample
- More often, we use technology

Take a RANDOM sample!

- Before the 2008 election, the Gallup Poll took a random sampleof 2,847 Americans. 52% of those sampled supported Obama
- In the actual election, 53% voted for Obama
- Random sampling is a very powerful tool!!!

- Pick 10 “random” numbers between 1 and 268. Write these numbers down.
(Note: When choosing a real sample, you should use technology to generate random numbers. This is simply for illustrative purposes in class.)

- Using the next slide, calculate the average number of letters in the words corresponding to your random numbers
- Place a dot below this average on the board

Fall 2011:

Spring 2012:

- Random samples have averages that are centered around the correct number
- Non-random samples may suffer from sampling bias, and averages may not be centered around the correct number
- Only random samples can truly be trusted when making generalizations to the population!

Think of tasting a bowl of soup…

- Population = entire bowl of soup
- Sample = whatever is in your tasting bites
- If you take bites non-randomly from the soup (if you stab with a fork, or prefer noodles to vegetables), you may not get a very accurate representation of the soup
- If you take bites at random, only a few bites can give you a very good idea for the overall taste of the soup

- In a simple random sample, each unit of the population has the same chance of being selected, regardless of the other units chosen for the sample
- More complicated random sampling schemes exist, but will not be covered in this course

- While a random sample is ideal, often it isn’t feasible. A list of the entire population may not be available, or it may be impossible or too difficult to contact all members of the population.
- Sometimes, your population of interest has to be altered to something more feasible to sample from. Generalization of results are limited to the population that was actually sampled from.
- In practice, think hard about potential sources of sampling bias, and try your best to avoid them

Suppose you want to estimate the average number of hours that Duke students spend studying each week. Which of the following is the best method of sampling?

- Go to the library and ask all the students there how much they study
- Email all Duke students asking how much they study, and use all the data you get
- Give a clicker question in STAT 101 and force every student to respond
- Stand outside the Bryan Center and ask everyone going in how much they study

- Sampling units based on something obviously related to the variable(s) you are studying
- Sampling only students in the library when asking how much they study, or sampling only students taking a statistics class
- “Today’s Poll” on fitnessmagazine.com asked “Have you ever hired a personal trainer?”. 27% of respondents said “yes” – can we infer that 27% of all humans have hired a personal trainer?

- Letting your sample be comprised of whoever chooses to participate (volunteer bias)
- People who chose to participate or respond are probably not representative of the entire population
- Emailing or mailing the entire population, and then making conclusions about the population based on whoever chooses to respond
- Example: An airline emails all of it’s customers asking them to rate their satisfaction with their recent travel

- The Federal Office of Road Safety in Australia conducted a study on the effects of alcohol and marijuana on performance
- Volunteers who responded to advertisements for the study on rock radio stations were given a random combination of the two drugs, then their performance was observed
- What is the sample? What is the population?
- Is there sampling bias?
- Will the results be informative and/or do you think the study is worth conducting?

Source: Chesher, G., Dauncey, H., Crawford, J. and Horn, K, “The Interaction between Alcohol and Marijuana: A Dose Dependent Study on the Effects of Human Moods and Performance Skills,” Report No. C40, Federal Office of Road Safety, Federal Department of Transport, Australia, 1986.

- Note: The original sources for the studies are provided and linked when possible - if interested in the details of the study, please check out the original article!

Data Collection and Bias

Sample

Sampling Bias?

Population

Other forms of bias?

DATA

- Even with a random sample, data can still be biased, especially when collected on humans
- Other forms of bias to watch out for in data collection:
- Question wording
- Context
- Inaccurate responses
- Many other possibilities – examine the specifics of each study!

- A random sample was asked: “Should there be a tax cut, or should money be used to fund new government programs?”
- A different random sample was asked: “Should there be a tax cut, or should money be spent on programs for education, the environment, health care, crime-fighting, and military defense?”

Tax Cut: 60%Programs: 40%

Tax Cut: 22%Programs: 78%

- Ann Landers column asked readers
“If you had it to do over again, would you have children?

- The first request for data contained a letter from a young couple which listed worries about parenting and various reasons not to have kids
- 30% said “yes”
- The second request for data was in response to this number, in which Ann wrote how she was “stunned, disturbed, and just plain flummoxed”
- 95% said “yes”

If we were to run the question all by itself in the newspaper with a request for responses, could we trust the results?

- Yes
- No

This would suffer from volunteer bias. We need a random sample.

Newsday conducted a random sample of all US adults, and asked them the same question, without any additional leading material

- 91% said “yes”
Do you think the true proportion of US parents who are happy they had children is close to 91%?

- Yes
- No

Because this is a random sample, the population proportion should be close to the sample proportion.

- In a study on US students, 93% of the sample said they were in the top half of the sample regarding driving skill
Svenson, O. (February 1981). "Are we all less risky and more skillful than our fellow drivers?" ActaPsychologica 47 (2): 143–148.

Always think critically about how the data were collected, and recognize that not all forms of data collection lead to valid inferences

- This is the easiest way to instantly become a more statistically literate individual!

- Read Section 1.2
- Complete the class survey (due Tuesday, 9/4)
- If you haven’t already…
- Get the textbook
- Get a clicker and register it
- Do Lab 0 TODAY