1. Types of Bias
Section 4.1A

2. Objective Purpose To introduce methods of sampling and types of bias.
To understand the importance of controlling bias in sampling to make better generalizations of populations.

3. Remember…. Population Sample
Consists of all objects that I wish to describe
Census – survey of the population
Sample
Subset of the population
Used to predict the population

4. Sample Survey
We often draw conclusions about a whole population on the basis of a sample. Choosing a sample from a large, varied population is not that easy.

5. Choose a sample Survey Define the population we want to describe.
Say exactly what we want to measure.
*A “sample survey” is a study that uses an organized plan to choose a sample that represents some specific population
Decide how to choose a sample from the population

6. Current Population Survey
Contacts 60,000 households each month
Produces monthly unemployment rate and a lot of other economic and social information
Population is defined as all U.S. residents (legal or not) 16 years of age and over who are civilians and are not in an institution such as prison.
Unemployed – if you are available for work and if you actually looked for work in the last 4 weeks.

7. Bad Sampling….
Sample that does not represent the population.

8. Convenience Sampling Choosing individuals who are easiest to reach.
Example: Questioning the 1st 100 people to come to the store
Example: Mall interview
What problems can you see?

9. Voluntary Response Sample
Consists of people who choose themselves by responding to a general appeal.
Those most likely to respond are the people with strong opinions, especially negative opinions.
Television call in polls

10. Example
The American Family Association (AFA) is a conservative group that stands for “traditional family values. It had a poll on its Web site – about same sex marriage in They had 60% of 850,000 people responded that they favored same-sex marriage. This did not support AFA’s position. What do you think happened?

11. What type of Sampling
A farmer brings a juice company several crates of oranges each week. A company inspector looks at 10 oranges from the top of each crate before deciding whether to buy all the oranges.

12. What type of Sampling
The ABC program Nightline once asked whether the United Nations should continue to have its headquarters in the United States. Viewers were invited to call one telephone number to respond “Yes” and another for “No”. There was a charge for calling either number. More than 186,000 callers responded and 67% said “No”.

13. Activity #1
Guess the length of my string.

14. Activity #2
I want to know the average size of the rectangles on this page.
Pick 5 rectangles that you believe are representative of all of the on the page.
Find the area of each of the 5 you chose.
Find the average area of all 5

15. Bias
The sampling method is biased if it systematically favors certain outcomes.
Exam Tip: Always tell which way it is biased.

16. Selection Bias
The method of selecting the sample systematically excludes some part of the population of interest.
Example – the rectangles lab
voluntary response

17. Measurement / Response
Method of observation tends to produce values that systematically differ from the true value in some way.
Measurement – improperly calibrated scale, the string activity
Response – Improperly worded questions

18. Nonresponse bias
Responses are not actually obtained from all individuals selected for inclusion in the sample.
Example – Failure to return polls

19. What type of Bias
Bill is assigned by his editor to determine what most Americans think about a new law that will place a federal tax on all modems and computers purchased. The revenues from the tax will be used to enforce new online decency laws. Bill, being technically inclined, decides to use an poll. In his poll, 95% of those surveyed opposed the tax. Bill was quite surprised when 65% of all Americans voted for the taxes.

20. What type of Bias
The United Pacifists of America decide to run a poll to determine what Americans think about guns and gun control. Jane is assigned the task of setting up the study. To save mailing costs, she includes the survey form in the group's newsletter mailing. She is very pleased to find out that 95% of those surveyed favor gun control laws and she tells her friends that the vast majority of Americans favor gun control laws.

21. What type of Bias
Large scale polls were taken in Florida, California, and Maine and it was found that an average of 55% of those polled spent at least fourteen days a year near the ocean. So, it can be safely concluded that 55% of all Americans spend at least fourteen days near the ocean each year

23. Simple Random Sample. Good sampling technique.
A simple random sample (SRS) of size n consists of n individuals from the population chosen in such a way that every set of n individuals has an equal chance to be the sample actually selected.

24. Let’s look at those rectangles again!
Let’s randomly pick 5 rectangles.
Press
Math
Prb
RandInt (1,100,5)
Find the area of the five rectangles that match the numbers you generated.

26. Choosing a SRS…using a Random Number Chart
Be specific about how you select
Indicate the stopping rule
Tell whether you sample with or without replacement
Use labels to identify subjects selected to be in the sample

27. We are planning an article on family-friendly places to stay over spring break at a nearby beach town. The editors intend to call 4 randomly chosen hotels to ask about their amenities for families with children. They have an alphabetized list of all 28 hotels in the town. 01
Aloha Kai 08
Captiva 15
Palm Tree 22
Sea Shell 02
Anchor Down 09
Casa del Mar 16
Radisson 23
Silver Beach 03
Banana Bay 10
Coconuts 17
Ramada 24
Sunset Beach 04
Banyan Tree 11
Diplomat 18
Sandpiper 25
Tradewinds 05
Beach Castle 12
Holiday Inn 19
Sea Castle 26
Tropical Breeze 06
Best Western 13
Lime Tree 20
Sea Club 27
Tropical Shores 07
Cabana 14
Outrigger 21
Sea Grape 28
Veranda

28. Activity
Which rectangle would you choose if you used the random number table? (Table D – back of the book)
How would you do it?

29. Homework
Page 226 (1-11) odd
Worksheet – Bias