1. Who do I ask, what do I ask them, what does that tell me?
Sampling and Surveys
Who do I ask, what do I ask them, what does that tell me?

2. The Cycle of Statistics
Population
Sample
Parameter
Statistic

3. Types of Sampling Simple Random Sample
Every member of the population has an equal chance of being chosen for the sample
Method
Create a sample frame
Assign a number to each individual in the sampling frame
Select only those whose numbers satisfy some rule

4. SRS
Example
There are 80 students enrolled in an introductory Statistics course; you are to select a sample of 5
Sampling frame
The roster of all students enrolled in the course
Label each student
Use a random number generator and choose the first 5 students from the list that match the random numbers. Ignore numbers not on the list and repeats.

5. Stratified Random Sampling
strata
Homogenous groups within the population
Method
Divide the population into strata
Take a SRS within each strata
Compare the results

6. Stratified random sampling
Example
What is the freshman opinion of the cafeteria food on campus?
Strata:
Men, Women
Sampling Frame:
A list of all enrolled students
Create two lists, one with males and one with females, label all of the individuals
SRS
Using random numbers choose 50 males, repeat for females.
Determine the male opinion and the female opinion

7. Cluster Sampling Cluster A representative group of the population
Method
Select one or multiple clusters at random and perform a census within each of them
Example
What is the freshman opinion of the cafeteria food on campus? (gender not an issue)
Problem: It can be difficult to track down the members chosen for the stratified sample
All Freshmen live in dorms which can serve as a cluster (assuming that there is no all male/female dorm)
Randomly choose 1 or 2 dorms at random and ask everyone in the dorm their opinion.

8. Multistage sampling Sampling schemes that combine several methods
Example
What is the freshman opinion of the cafeteria food on campus? Gender matters.
Perform a cluster sample as mentioned
Many dorms house females and males on separate floors.
Randomly choose a dorm (cluster) and then randomly choose a few floors with only men or women (strata) and then interview every member of that floor (cluster).

9. Systematic Sampling A sample that selects individuals systematically.
Ex. Every 10th person
Method
Create a sampling frame
Choose a random starting point
Choose every nth individual from that starting point
Benefits
Cheap and easy
Need to make sure the sampling frame has no associations. ie. Every 5th individual is not the same.

10. Bias and Problems with sampling
Voluntary Response sampling
Bad sampling frame
Telephone book misses people with only cell phones, homeless, prisoners
Undercoverage
Telephone surveys
If calling at dinner time will miss people that eat out often
Convenience sampling
Non respondents
Those that don’t respond may differ significantly from those that do respond.
Response Bias
from poorly designed questions or poor interviewer