1. Chapter 12 part 1
Sample surveys

2. Measuring an entire population is impractical, if not impossible, so we settle for examining a sample. We want the sample to be random.

3. How can we ask questions of a small group of people in hopes of learning something about the entire population?
SAMPLE SURVEYS

4. 2012 Presidential Election
Opinion polls are a popular type of sample survey that are used in politics.
2012 Presidential Election

5. Important things to remember:
A sample needs to be random!
Randomness helps prevent bias.
The larger the sample, the more precise the results.

6. What is wrong with each of the following statements?
It is always better to take a census than to draw a sample.
Stopping students on their way out of the cafeteria is a good way to sample if we want to know about the quality of the food there.
It is extremely difficult to reach all members of a population and it can take so long that the circumstances change.
The sample is probably biased. Students who don’t like the food might not eat in the cafeteria.

7. What is wrong with each of the following statements?
3) We drew a sample of 100 from the 3000 students in a school. To get the same level of precision for a town of 30,000 students, we’ll need a sample of 1000.
4) The true percentage of Statistics students who enjoy doing the homework is called a “population statistic.”
The sample size of 100 would be equally precise for each population. It’s the sample size that matters, not the population size.
Of course ALL Statistics students enjoy doing their homework.  But it would be called a population parameter.

8. How do we get a sample that is representative of the population?
A Simple Random Sample (SRS) is a sample drawn in a way that each combination of people has an equal chance of being selected.

9. Example of a SRS:
There are 80 students in a Statistics course and you are to select a sample of 5 students using these random digits: How might you choose the 5 students?

10. One solution would be to number the students alphabetically from 01 to 80, and then take two digits at a time from the list of random numbers.

11. Stratified random sample Cluster sample Systematic sample
There are other (often more convenient) ways of gathering a sample in addition to a SRS:
Stratified random sample
Cluster sample
Systematic sample
Convenience sample

12. Stratified Random Sample
The population is divided into several subpopulations (called strata) and random samples are drawn from each stratum.

13. Cluster Sample
Entire groups, or clusters, are chosen at random. Every member of the selected group is a part of the sample.

14. Systematic Sample
Arrange the members of the population, randomly select a starting point, and choose every 8th member in the list, or every 30th member in the list, or every 100th member, etc.
Choose the number based on the sample size needed.

15. Convenience Sample
Consists of individuals that are conveniently available.
Not advised! They are not usually representative of the population!

16. a) Systematic; b) Stratified; c) Simple Random; d) Cluster
We want to survey a random sample of the 300 passengers on a flight from San Francisco to Tokyo. Which sampling method is described below?
Pick every 10th passenger as people board the plane.
From the boarding list, randomly choose 5 people flying first class and 25 of the other passengers.
Randomly generate 30 seat numbers and survey the passengers who sit there.
Randomly select a seat position (right window, right center, right aisle, etc.) and survey all passengers sitting in those seats.
a) Systematic; b) Stratified; c) Simple Random; d) Cluster

17. Jelly Blubber Activity
Skim over the jelly blubbers and make a guess at the average length. Write down your guess.
Choose a sample of 10 jelly blubbers that you think are representative of the whole group. Write down the number and length of each and calculate the mean.
Use a random number generator to repeat step 2 with a SRS.

18. Today’s Assignment: Add to HW: p. 288 #1–4, 7–10, 15–18, 20
Read Chapter 12