Transcript Document 7456693

Stat 512
Day 2: Designing Experiments
Leftovers from Tuesday
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Questions on syllabus?
Repeating the question
Blackboard problems?
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Sooner turn in, sooner get feedback?
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Microwave popcorn factory
 Two categorical variables
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80%
60%
52
43
airw ay obstructed
40%
20%
0%
airw ay not obstructed
6
low
15
high
Level of Exposure
level of exposure, whether airway obstruction
Graphical summary: segmented bar graph
Numerical summary: difference in conditional
proportions
Not able to draw cause-and-effect conclusions
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100%
Percentage
Last Time
Figure 1: Comparison of Airway Obstruction
from Level of Exposure
Other differences between the two groups that
might explain the higher airway obstruction rates in
high exposure group?
Not able to generalize to all microwave plant
workers
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“Healthy worker effect”
100%
Percentage
Last Time
80%
60%
Control
Lung cancer patient
40%
20%
0%
none
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Smoking and lung cancer
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light
mod
heavy excess chain
Level of Smoking
Two categorical variables (amount of smoking and
with or without disease)
Graphical summary: segmented bar graph
Numerical summary: difference in conditional
proportions
Can’t draw cause and effect conclusion
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Could be some other difference (diet) between the EV
groups that explains higher lung cancer rates with more
smoking
Last Time
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Generalizing from sample to population?
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Reasonable to conclude 605/(605+780), or 44%,
of all males of similar ages and economic status
have lung cancer?
Measurement issues
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Relying on recall…
Know they are sick…
Second famous smoking study
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Hammond and Horn (1958)
Find 12,000 healthy men, complete a
questionnaire on smoking habits, had 22,000
American Cancer Society volunteers follow
them for 44 months to see whether they die
from lung cancer
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Advantages? Disadvantages?
Practice Problems
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Identifying variables
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Supreme Court Justices:
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Qualitative: gender, party
Quantitative: age, number of yes votes
Not OK:
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number of republicans
Practice Problems
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Victims of violence
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OU: people (not “number of victims”)
EV: whether abused
RV: whether commit crime
Not ok in defining variables:
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Those who…
Number abused…
Whether abuse leads to violent crime…
(c) Mostly to spur discussion…
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Which variable was “controlled” by the researchers…
More practice:
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The book Day Hikes in San Luis Obispo County by
Robert Stone gives information on 72 different hikes
that one can take in the county. For each of the 72
hikes, Stone reports the distance of the hike (in
miles), the anticipated hiking time (in minutes), the
elevation gain (in feet), and the region of the county
in which the hike can be found (North County, South
County, Morro Bay, and so on, for a total of eight
regions).
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Observational units?
Types of variables
More Practice: Hiking in SLO
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Are these legitimate variables?
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Longest hike in the book
Those hikes in the North County region
Proportion of hikes with an elevation gain of more
than 500 feet
Is hiking time related to elevation gain?
Example 1: Near-sightedness and Night
Lights
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“Myopia and ambient lighting at night,”
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Quinn, G.E., Shin, C.H., Maguire, M.G. and
Stone, R.A.
Nature, 399:113-114, 1999.
Example 1: Myopia and Night Lights
Far-sighted
Normal
Near-sighted
Room light
Night light
Darkness
12
22
41
39
115
78
40
114
18
100%
90%
80%
70%
60%
Near-sighted
50%
Normal
40%
30%
Far-sighted
20%
10%
0%
Room light
Night light
Darkness
Example 1: Myopia and Night Lights
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Base rate: .286
Conditional Proportions
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Room light: .55 myopia, .16 hyperopia
Night light: .336 myopia, .168 hyperopia
Darkness: .105 myopia, .232 hyperopia
Convincing evidence that using more light in
the child’s room causes a higher rate of
myopia?
Terminology: Confounding variable
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Has an influence on the response, but its
effects cannot be separated from those of the
explanatory variable
room light
parents with bad eyes
Compare
darkness
parents with good eyes
eye-sight
Example 2: Have a Nice Trip
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Can instruction in a recovery strategy
improve an older person’s ability to recover
from a loss of balance?
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12 subjects have agreed to participate in the study
Assign 6 people to use the lowering strategy and
6 people to use the elevating strategy
Similar amounts of men in both groups?
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Proportion of group 1 that are male – proportion of group
2 that are male
What would we like to be true about these proportions?
Investigation 1-7: Have a Nice Trip
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Take 12 index cards
Put one of the 12 names on each card
Shuffle the cards and deal out 6 to learn the
lowering strategy and the other 6 to learn the
elevating strategy
What is the number of males in each group?
The male proportion in each (out of 6)?
What is the difference in these 2 proportions?
Pool Results
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Quantitative variable
Different type of graph…
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What are the observational units and variable in
this graph?
Long-term pattern?
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Use applet to repeat the process a large
number of times
Open IE, double click on “Dr. Beth Chance” >
“Statistical Methods” > “Stat 512 Java
Applets” > “Randomizing Subjects
The applet mimics exactly what you did with
the index cards.
Effect of randomization
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If you randomly assign subjects to the
groups, what is generally true about the
groups?
If after imposing the treatment, you later
observe a difference between the groups, to
what can you attribute that difference?
Moral
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Randomization equalizes variables between
groups
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Should not have potentially confounding variables
If later (after treatments) observe a difference
between groups, feel comfortable attributing that
difference to the explanatory variable
Remaining question
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Will always be some difference, by chance
How big does this difference have to be?
Example: Friendly Observers
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“The trouble with friendly faces: Skilled
performance with a supportive audience,”
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Butler, J. L., and Baumeister, R. F.
Journal of Personality and Social Psychology, 75:
1213-1230, (1998).
To do
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By Friday, noon
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Turn in HW 1 (4 problems)
By Tuesday
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Preview the Friendly Observers example
(complete questions (a)-(e))
Submit PP 2 in Blackboard
Pick and start reading one of the 3 articles