1. Chapter 5 Sampling and Surveys
5.1 Samples, Good and Bad

2. Bias Samples Sample is a small group from a larger population.
Design flaw of a statistical study that systematically favors certain outcomes.
Systematically favoring one outcome over another
Samples
Sample is a small group from a larger population.
Representative samples have the same characteristics of the larger population

3. BIASED SAMPLES: Voluntary Response Sample
Individuals choose to become part of the sample by responding to a general request.
Very biased in that many reply with extreme or strong opinions on the subject matter.
Not a representative sample since not all people would choose to respond.
EX: Call-in, write-in, asking people at the mall to stop, etc.

4. Convenience Sample Selection of individuals that are easily obtained.
Biased
Not a representative sample.

5. Practice: p. 205 #2-4
5.2 An online poll (a) Voluntary Response Sample (b) The sample is limited to those that read Parade, those that have access to online services, and were willing to go to the website to vote. The 85% is probably higher than the true percent of all adults who believe that cell phone use while driving should be banned. People that are for the ban on cell phones would probably have stronger feelings on the subject and be more likely to go online to complete the survey.

6. 5.3 Sleepless nights
(a) Convenience Sample
(b) Since the student chose the first 100 to arrive these students had probably had a good night sleep, awoken refreshed, and made it to school early. The average is probably higher than the true average since those that did not sleep well or went to bed late probably hadn’t made it to school yet.
5.4 Hand-washing habits
(a) When asked the question most people would not admit to not washing their hands.
(b) The observers did not want to bias the study by letting the people be aware of being watched. If the people knew they were being studied they might behave in a different manner.

7. Unbiased Sample Simple Random Sample (SRS)
Sample of size n from a population chosen in such a way that:
Every individual of the population has the same chance of being selected
Every possible sample of size n has the same chance of being selected.

8. Example: 4 Students need to be selected to represent the class in Statistics Conference in Cancun during Spring Break.
Put everyone’s name in hat, mix it up, and select 4 names.
Put all the boy’s names in one hat and all the girl’s names in a second hat. Randomly select 2 boys and 2 girls.

9. How to take a SRS:
Table of Random Digits (table B)-
A long string of digits (0-9) with the following properties:
Each digit in the table is equally likely to be any of the 10 digits
The entries are independent of each other. So knowing one digit doesn’t affect what the next one is.

10. HOW TO USE THE TABLE:
1) LABEL
* Label each individual in your population with a #
* Each # given must have the same amount of digits
2) TABLE
* Go to the table, read across in groups that are the same size as the numbers you assigned

11. How to write instructions for SRS:
Label each _______ with #__ -- __
Use TRD, read across, every ____ digits represent a ______.
Ignore #’s ________, and repeats
The first _____ numbers are our sample

12. EXAMPLE:
Take a sample 5 from the following list. Start at line 131 in the table.
Smith Jones Holloway
DeNizzo David Adams
Schaefer Gray Capito
Meyers Gingrich Card
Dietrich Moreland Hall
Walsh Whitter Jordan

13. Instructions Assign 01-18 to each of the people
Read across the TRD, every 2 digits represents a person
Ignore 19-99,00 and repeats
The first 5 numbers are our sample

15. Practice - p. 209 #7 - 9 7. Apartment Living Instructions:
Label each apartment complex with 01-33
Use TRD, read across every 2 digits is an apartment complex.
Ignore 00,34-99 and repeats
First 3 complexes are our sample
Line 117:
Complexes: Fairington, Waterford Court, Fowler

16. 8. How do random digits behave?
False
True
9. An election day sample
(a) Instructions:
Label each precinct with #001 – 440.
Use TRD and read across every 3 digits = one precinct.
Ignore 000, and repeats.
The first 25 precincts are our sample

17. Precincts: 395, 020, 118, 167, 300, 360, 241, 065, 188, 365

18. Random Sampling on the Calculator

19. HW
p. 210 #10
p. 217 #15, 16
p. 218 #21, 22