Chapter 8
•
A
sample survey
selects a
sample
from the
population
of all
individuals
about which we
desire information. We base conclusions about the
population
on data from the
sample
. It
is important to specify exactly what
population
you are interested in and what
variables
you will measure.
•
The
design
of a
sample
describes the method used to select the
sample
from the
population
.
Random sampling
designs use chance to select a
sample
.
•
The basic random sampling design is a
simple random sample
(SRS).
An SRS gives
every possible
sample
of a given size the same chance to be chosen.
•
Choose an SRS by labeling the members of the
population
and using
random digits
to
select the
sample
. Software can automate this
process
.
•
To choose a
stratified random sample
,
classify the
population
into
strata,
groups of
individuals
that are similar in some way that is important to the response. Then choose a
separate SRS from each stratum.
•
Failure to use random sampling often results in
bias
,
or systematic errors in the way the
sample
represents the
population
.
Voluntary response samples
,
in which the respondents
choose themselves, are particularly prone to large
bias
.
•
In human
populations
, even random
samples
can suffer from
bias
due to
undercoverage
or
nonresponse
,
from
response
bias
,
or from misleading results due to
poorly worded
questions.
Sample
surveys must deal expertly with these potential problems in addition
to using a random sampling design.
•
Most national
sample
surveys are carried out by telephone,using
random digit dialing
to
choose residential telephone numbers at random. Call screening is increasing
nonresponse
to such surveys, and the rise of cellphoneonly households is increasing
undercoverage
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 Fall '11
 
 Nonresponse

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