Transcript Chapter 4 (Sections 1 & 2) PowerPoint

GATHERING DATA
Chapter 4
4.1 Experiment or Observe?
Population and Samples
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Population: Subjects of
interest
Sample: Subset for whom
we have data
Often want answers
about large group but
can’t measure all, so a
subset is chosen
Use statistical techniques
to infer conclusions
Observational Study
Merely observe values of response and
explanatory variables without doing anything
to the subjects
Ex. Cell Phone Study 1 (Page 155)
Cell Phone Study 2 (Page 155)
Sample Survey
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Select sample
and interview
Observational
study
Census is survey
of entire
population
Experiment
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
Assign subjects to certain experimental conditions
and observe outcomes of the response variable
The experimental conditions, which correspond to
assigned values of the explanatory variable are
called treatments
Ex. Cell Phone Study 3 (Page 155)
Experiments and Observational Studies
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Experiment reduces lurking
variables and thus outside
influences
Experiments establish cause
and effect, unlike
observational studies
Some experiments
impractical because of
ethics, time, money, etc.
Exs. # 4.2, 4.8 Page 162
4.2 What are Good and Poor Ways to Sample?
Sampling Frame & Sampling Design
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Sampling frame – list of
subjects in (hopefully
total) population
Sampling design
determines how sample
is selected
Simple Random Sampling
Random Sampling – best
way to get representative
sample
 Simple Random Sample –
each possible sample of set
size n has equal chance of
being selected
Ex. 4 Page 164
Simulate with Calculator/CD

Choosing Random Numbers
Pg. A6 of text
1.
2.
3.
Number subjects from 1 to n
Select numbers from random number table or
random number generator (calculator or
computer)
Include subjects with random numbers selected
Margin of Error for Population Percentages

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Margin of Error – how well sample predicts
population
For a random sample with n subjects, the margin
of error is approximately
1
n
100%
Ex. A survey result states: “The margin of
error is plus or minus 3 percentage points”
Convenience Samples: Poor Ways to Sample

Convenience Sample:
survey sample that’s
easy to get
 Unlikely to represent
population
 Often severe biases
 Results apply only to
observed subjects
Convenience Samples: Poor Ways to Sample
Volunteer Sample:
most common
convenience sample
where subjects
volunteer – not
representative
Types of Bias in Sample Surveys
Bias: Favoring parts of population
1. Sampling Bias: from sampling method (e.g.,
nonrandom samples)
2. Nonresponse bias: some subjects cannot be
reached or decline
3. Response bias: subject gives incorrect response or
question is misleading
Exs. # 4.24, 4.29