Transcript 5.2 Designing Experiments

Section 5.2
Part 1
Explanatory Variable
 Any variable that explains the response variable. Often
called an independent variable, predictor variable, or
factor.
Response variables
The outcome of a study. A variable you would be interested in
predicting or forecasting. Often called a dependent variable or
predicted variable.
Advertising: What are the results of repeated exposure to an
advertising message?
 The treatment in the study of repeat exposure to advertising.
Combining the levels of the two factors forms six treatments.
Basic Principles of Statistical Design
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Control the effects of lurking variables on the
response, most simply by comparing two or more
treatments.
Replicate each treatment on many units to reduce
chance variation in the results.
Randomize – use impersonal chance to assign
experimental units to treatments.
Control
 A simple experiment in a controlled laboratory environment
often has this form.
 Treatment---› Observed Response
 In field experiments, this design can be too simple
 Placebo effect: many patients respond favorably, to any
treatment, even a placebo. A placebo is a dummy treatment.
 Control group: The group of subjects that receive the dummy or
placebo treatment.
Caution!!!
 Don’t confuse control and control group.
 Control refers to the overall effort to minimize
variability in the way experimental units are
obtained and treated.
Replication
 Even with control, there will still be natural variability
or systematic differences between the groups.
 If we assign many units to each group, the effects of
chance will average out.
 Use enough subjects to reduce chance variation.
Randomization
 The use of chance to divide experimental units into
groups.
Cell phones and driving: Does talking on a hands-free cell phone device distract
drivers?
Completely Randomized Design
When all experimental units are allocated at random
among all treatments, the experiment is said to have a
completely randomized design.
Helper-Hinderer
In a study reported in a November 2007 issue of Nature, researchers
investigated whether infants take into account an individual’s actions
towards others in evaluating that individual as appealing or aversive,
perhaps laying the foundation for social interaction (Hamlin, Wynn, and
Bloom, 2007). In one component of the study, sixteen 10-month-old
infants were shown a “climber” character (a piece of wood with “google”
eyes glued onto it) that could not make it up a hill in two tries. Then they
were shown two scenarios for the climber’s next try, one where the
climber was pushed to the top of the hill by another character (“helper”)
and one where the climber was pushed back down the hill by another
character (“hinderer”). The infant was alternately shown these two
scenarios several times. Then the child was presented with both pieces of
wood (the helper and the hinderer) and asked to pick one to play with.
The color and shape and order (left/right) of the toys were varied and
balanced out among the 16 infants.
http://www.youtube.com/watch?v=anCaGBsBOxM