1. Stat 100 Jan. 21 Read Chapter 4, Try problems 1,2, 5,7,9,14,15,17

2. Chap 4 Thought Question 2
A random sample of 1600 Americans is asked if they regularly watch a certain TV show. 24% says “yes.”
How close do you think this percentage is to the percent of the entire country who watch the show? Within 30%? 10%? 5%? 1% ?

3. December 2001 Gallup Poll
1,005 adult Americans randomly sampled

4. Margin of Error
Likely maximum size of sampling error for 95% of all random samples of a certain size.

5. Approximating Margin of Error
n = sample size
n= 1,600 people

6. Interpreting Margin of Error
For 95% of all random samples of the given size, the difference between the sample and population percents is less than the margin of error

7. Confidence Interval
Interval of values that is likely to capture the unknown population percent
95% confidence interval is: sample percent ±margin of error

8. Example
Gallup Poll, Dec of n = 1005 adults found that 26% smoked cig’s in last week
Margin of error given as 3%.
With 95% certainty, we can say that between 23% and 29% of U.S. adults smoke cigarettes.
Calculated as 26% +/- 3%

9. Ladies Home Journal Sex Survey
Intended population - American married women
Questionnaire about sexual behavior published in an issue of the magazine
40,000 respond
What will bias the results?

10. Biasing Factors “Sampling frame” is wrong.
Sampling magazine readers rather that all American married women.
Sampling method is bad - using self-selected volunteers
False information
Can’t know if respondents are really married women.

11. General Difficulties and Disasters
Using wrong sampling frame
Not reaching a particular type of person even though they were selected
Non-response and relying only on “volunteer” response
Self-selected samples

12. Book Ch. 4 Thought Question 5
Most Gallup Polls sample about 1,200 individuals. How do you think they select people?

13. Some Good Sampling Methods
Simple Random (like a lottery)
Stratified
Systematic
Cluster

14. Simple Random
Any group of the specified size has the same chance to be the sample

15. Stratified Method
Divide population into important subgroups that should be represented
Randomly sample from each subgroup.
Provides most direct control over representativeness.

16. Systematic Method
Systematically select from a list or sequence of items.
For example, take every 50th name.
Or, check quality of every 200th item on assembly line.

17. Cluster Method
“Clusters” are naturally occurring living groups (like dorms)
Select some clusters to be in the sample and other to be out
Usually, sample individuals within the selected clusters.