Download Section 4.1 Second Day Inference from Sampling, What Can Go

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Chapter 4: Designing Studies
Section 4.1
Samples and Surveys
The Practice of Statistics, 4th edition – For AP*
STARNES, YATES, MOORE
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Random = Good.
 Each
student should take an SRS of 20 chips
from the bag. Record the proportion of chips
that are red. Return the chips to the bag.
Make a dotplot of the class’s results.
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Trust
 Why
can we trust samples that are random?
 The results obey the laws of probability.
Eventually, we’ll be able to say things like, “We are
95% confident that…”
 For
now, one point is worth noting:
 LARGER RANDOM SAMPLES GIVE BETTER
INFORMATION ABOUT THE POPULATION THAN
SMALLER SAMPLES.
Surveys: What Can Go Wrong?
Most sample surveys are affected by errors in addition to
sampling variability.

Good sampling technique includes the art of reducing all
sources of error.
Definition
Undercoverage occurs when some groups in the population
are left out of the process of choosing the sample.
Nonresponse occurs when an individual chosen for the sample
can’t be contacted or refuses to participate.
A systematic pattern of incorrect responses in a sample survey
leads to response bias.
The wording of questions is the most important influence on
the answers given to a sample survey.
Sampling and Surveys

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 Sample
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Sampling errors vs. Non-sampling
errors
Sampling Errors

A sampling error is a mistake
made in the process of taking a
sample that could lead to
inaccurate information about the
population.

Undercoverage is a sampling
error.
 Undercoverage occurs when
the “sampling frame” doesn’t
list every member of the
population.

Poor sample designs lead to
sampling errors.
Non-sampling Errors

These are errors that don’t
stem from the sampling process
– they could even result from a
census!

Examples of non-sampling
errors:

Nonresponse
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Response bias

Wording of questions
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Do you brush your teeth at least
twice a day and floss every day?
News
flash: most people don’t
answer this question honestly.
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Two Questions – wording matters!

Should illegal immigrants be
prosecuted and deported for
being in the U.S. illegally, or
shouldn’t they?

69% favored deportation.

Should illegal immigrants who
have worked in the United
States for two years be given a
chance to keep their jobs and
eventually apply for legal
status?
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62% said yes.