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Producing Data
Lab #3
Reading correlation table: Bottom of p.
52 and top of page 53 of Sorenson
Reading regression output to construct
your equation: Sorenson p. 55. You
need information in “coefficients” table
– table 5.2
Producing Data
SAMPLING
Exploratory Data Analysis
the use of tables and graphs to aid in
describing of quantitative data
Conclusions pertain to specific data
What happens when we wish to answer a
question about large groups of individuals
We must collect data in such a way that will
allow answering such questions
E.g. What are the attitudes of American population
towards gambling?
Census
Samples
Producing Data
Two ways of gathering sample data
Observational Study
Observes individuals and measures variables
without the attempt to influence the responses.
Ask students how they feel about cumulative final
exams – sample survey/opinions
A biologist studies the behavior of lobsters through
observation of such behavior
Producing Data
Two ways of gathering sample data
Experiment
Imposes some treatment on individuals in
order to observe response
To understand cause and effect
Involves intervening not just observing
Medical experiment where 20,000 male physicians
over age 40 were used to test the theory that aspirin
reduces risk of heart attack
Producing Data
Confounding
Observational studies should not be
used in the study of effect of one
variable on another
Explanatory variable is confounded with
lurking variables.
Two variables are confounded when
their effects on a response variable
cannot be distinguished from each other
Moderate use of alcohol is associated
with better health. Some studies
suggest that drinking wine rather than
beer or spirits confers added health
benefits. Confounding?
Producing Data
But, people who prefer wine are different from
those who drink mainly beer and stronger
stuff. Wine drinkers eat more fruit and
vegetables and less fried foods. Their diets
contain less fat, less cholesterol, and also less
alcohol. They are less likely to smoke.
Finally, wine drinkers as a group are richer
and better educated than people who prefer
beer or spirits.
Conclusion from study: The apparent health
benefits of wine compared with other
alcoholic beverages, as described by other,
may be a result of confounding by dietary
habits and other lifestyle factors.
Designing Samples
Sample Design – method used to choose the
sample from the population
Sampling design practices are extremely
important as improper designs may render
experimental results to be misleading or
invalid
TYPES OF SAMPLING
NON-PROBABILITY SAMPLES
VOLUNTEER SAMPLING
Sample based on individuals who volunteered to
participate in a study – choose themselves for study
CONVENIENCE SAMPLING
Choosing individuals for study based on availability
Individuals are easiest to reach
Corner interviews; call-in telephone polls; mall surveys,
etc..
Samples open to bias – sampling bias –
systematically favors choosing certain individuals for
samples, and thus favors certain outcomes or
conclusions from the data
TYPES OF SAMPLING
PROBABILITY SAMPLES
Attempt to minimize bias
1.
SIMPLE RANDOM SAMPLE (SRS)
Consist of individuals from a population chosen in
such a way that every set of n individuals has an
equal chance to be the sample actually selected
Srs gives each individual an equal chance of being
chosen
Srs gives each sample an equal opportunity of being
chosen
SAMPLING FROM A FINITE POPULATION
TYPES OF SAMPLING
PROBABILITY SAMPLES – samples chosen by
chance
1.
SIMPLE RANDOM SAMPLE (SRS)
IDEA: to choose our sample by drawing names
from a hat.
Let computer software randomize via TABLE
OF RANDOM DIGITS
A table of random digits is a long string of the
digits 0 through 9 with these properties:
1.
•
•
2.
•
•
Each entry in the table is equally likely to be
any of the 10 digits
No value produced by a random number
generator should be any more likely than any
other
Probability distribution of numbers produced
must be uniform
The entries are independent of each other.
There should be no possibility of predicting the
next number before it appears
In random samples, each individual has an
equal chance of being chosen into the sample
TYPES OF SAMPLING
Chose a simple random sample of 5
individuals from the following list of
students
Adam
John
Anna
Trisha
Oscar
Catherine
Donna
George
Michael
Paul
Maria
Robert