Austin Cole
February 16, 2010
Outline
I. Sampling
a. Bad Sampling Methods
b. Random Sampling
II. Experiments
III. Applying Sample to a Population
IV. Simulations
V. Confidence Intervals
VI. Discussion
Sampling
Population- entire group of individuals about
which we want information
Sample- part of population from which
information is collected
Unemployment
Monthly unemployment rate based on survey
of 60,000 households
Define population
Define unemployed
Final percentage
"Labor Force"
Unemployed
Not looking
for work
Employed
Bad Sampling Methods
Convenience sample-sample of easiest to
reach members of population
Bias-systematically favoring a certain outcome
Voluntary Response Sample-people choose to
respond to a general appeal
Simple Random Sampling
Every individual in population has equal
chance to be sampled
Table of random
digits
Cautions about Sample Surveys
Undercoverage-group of the population is left
out when choosing sample
Nonresponse-individual chosen doesn’t
participate
Wording of questions
Experiments
Observational Study
Experiment-imposes some treatment on
individuals to observe their responses
Confounding variables-variable whose effects
cannot be distinguished
Control group
Randomized Comparative Experiment
Online vs. classroom
courses
Random Sampling Exercise
1.Starting on line x, read 2-digit groups until
you have chosen 6 restaurants.
2.Ignore groups not in the range and ignore
any repeated labels.
Starting at line 105: 07, 19, 14, 17, 13, 15
Thinking about Experiments
Placebo effect
Double-blind
experiment
Prospective
studies
From Sample to Population
Statistical inference-using fact of a sample to
estimate about whole population
Parameter-fixed number that describes
population
Statistic-number that describes a sample
Sampling Distribution-distribution of values
taken by the statistic in all possible samples of
the same size from the same population
Simulation
Assessing simulations
Shape
Center-mean of sampling distribution (g)
Spread-standard deviation of sampling
distribution
g(1- g)
n
Confidence Intervals
Percent of all samples will produce an interval
containing the true population parameter
68-95-99.7 Rule
Margin of error for 95% confidence interval:
2
ĝ(1- ĝ)
n
95% Confidence Interval
Exercise
A Gallup poll asked a random sample of
1785 adults if they attended church or
synagogue in the last 7 days. Of the
respondents, 750 said yes. Find the 95%
confidence interval.
ĝ=.42
ĝ(1- ĝ)
n
=.023
95% Confidence Interval: .376 to .466
Discussion
In real world examples, what are some uses of
knowing the spread/standard deviation?
Other uses/applications for this information?
Homework
Problems:
9,38,44a
(7th edition)