1. 4.2 Day 3

2. Answers to Practice AP Problem
No, not appropriate. Instead, randomly assign one individual in each block to receive the new shampoo, while the other individual in each block receives the old shampoo

3. Plan for Today Practice AP Problem New Material (short)
Introducing the First Semester Project

4. Three Principles of Experimental Design
We have already talked about these separately to some extent
CONTROL: control for confounding variables that might bias your results
Control Group
Double Blind (if possible)
RANDOM ASSIGNMENT: use random assignment to choose who receives a particular treatment
Even if using a blocked or matched pairs design
REPLICATION: Use enough individuals/subjects that the results will not be due to random chance
In other words, bigger sample = better

5. Example Think back to the “Pregnant Blood” example from last class
How did the researchers use each of these three principles of experimental design?
CONTROL:
Control group (“regular” blood)
Double-blind (nobody knew which blood they were getting)
Random Assignment:
Within each gender, blood received was randomly assigned
Replication:
Had over 30,000 subjects—a very large study
Results were statistically significant
Presumably if the same experiment was conducted again, results would be similar

6. Statistical Significance

7. Statistical Significance
Think back to the airline pilot example that we did at the beginning of the semester
We concluded that there WAS a realistic possibility of the result occurring just by random chance
So our result was NOT statistically significant
We therefore (hypothetically) advised the male pilots not to sue the airline

8. Outlining/Describing an Experimental Design
You may be asked to do this on the AP Test (and on your HW)
You can do it in sentences if you like
No problem with this, just make sure you be very careful and detailed
Describe how people are randomly assigned, what your treatment(s) are, what happens to the control group, what you do afterwards, etc.
A potentially easier way is to make a diagram like we’ve seen in previous classes (see next slide)

9. Blood from Pregnant
Males
Random Assignment
Compare
Other Blood
Subjects
Blood from pregnant
Compare
Random Assignment
Other blood
Females

11. Drawing Conclusions Sample is representative of population
Sample is not representative (convenience sample or voluntary response sample)
Treatment was assigned randomly (experiment)
Can draw conclusions about BOTH the population AND causality
Can draw conclusions ONLY about causality (for the individuals in the sample only)
Treatment was not assigned randomly (observational)
Can ONLY draw conclusions about the population (the relationship between x and y in the population)
Cannot draw conclusions about either the population or causality

12. Project You must address one of the following questions:
Can the wording of the question create response bias?
Do the characteristics of the interviewer create response bias?
Does anonymity change the response to sensitive questions?
Does providing extra information affect the responses?
Does manipulating the answer choices affect the response?

13. Wording of the question
This one we’ve talked about before
An example from last year:
Some subjects would receive the question: “would you like to get a puppy?”
While others received the question: “Would you like to have to take care of a puppy?”
Their hypothesis was that these two versions would receive different response—a greater proportion of people would say yes to the first version than the second

14. Characteristics of the Interviewer
Another example from last year:
A group gave a survey to freshman females asking the following question: “are you more attracted to males with long hair or short hair?”
The question remained the same for all subjects. What changed was who handed them the survey
One of the group members had long hair, and the other had short hair
Their hypothesis was that the females would be more likely to choose the type of hair that their interviewer had

15. Anonymity To me, this is the least interesting option
But it is an option nonetheless
The idea here is to ask individuals a sensitive question (I must approve the question, remember!)
Randomly choose some individuals for whom the result is anonymous (and tell them this)
And some for whom it is not anonymous
See if there is a difference between the responses for the two groups

16. Extra Information An example from last year:
“Would you kill a bee if it landed on you?”
“Given that bees pollinate our food and are essential for human life, would you kill a bee if it landed on you?”
Their hypothesis was that people receiving the second version would be less likely to say yes

17. Manipulating the answer choices
Another less exciting one in my opinion
“If you had to guess, how far do you think the moon is from earth?”
240,000 miles
510,000 miles
740,000 miles
920,000 miles
120,000 miles
200,000 miles
290,000 miles

18. Designing your Project
You have about 10 days to come up with your idea (though you’re welcome to start talking to partners and developing a plan today)
You will submit your proposal to me on Nov 2/3
This is 15% of your project grade, so put some thought and effort into it
Things to think about:
What is your population?
How to sample individuals from that population
The wording of your survey questions
How to assign who gets which version of your survey
Your hypothesis or hypotheses