4.2 Day 3 10.23.2017

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

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

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

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

Statistical Significance

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

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)

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

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

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?

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

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

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

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

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

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