Chapter 11
Sample Surveys
How do we gather data?
•
•
•
•
Surveys
Opinion polls
Interviews
Studies
– Observational
– Retrospective (past)
– Prospective (future)
• Experiments
Population
• the entire group of
individuals that we
want information about
Census
• a complete count of the
population
Why would we not use
a census all the time?
1)
2)
3)
4)
Not accurate
Very expensive
Perhaps impossible
Look at the U.S. census – it
has a huge
amount
ofwould
error in
If using destructive
sampling,
you
Since
census
ofknow
any
Suppose
you
it; taking
plus
it awanted
takes
a to
long
to
destroy population
•
•
•
population
takes
time,
censuses
the
average
weight
of the
compile
the
data
making
much
Breaking strength of soda bottles
arethe
VERY
costly
to do.
white-tail
deer
population
in
of
data
obsolete
by the
Lifetime of flashlight
batteries
Texas – would it be feasible to
Safety ratings for carstime we get it.
do a census?
Sample
• A part of the population that
we actually examine in
order to gather information
• Use sample to generalize to
population
Sampling design
• refers to the method
used to choose the
sample from the
population
Sampling frame
• a list of every
individual in the
population
Simple Random
Suppose we were to take an SRS of
Sample
(SRS)
100
EHS students
– put each
Not only does each student has the
students’
name
in a hat. from
Then the
•same
Consists
of
n
individuals
chance to be selected – but every
randomly select 100 names from the
possible
group chosen
of 100 students
has
the
population
in
such
a
way
hat. Each student has the same
same chance to be selected!
chance to be selected.
that
Therefore, it has to be possible for all
100 students to be juniors in order for
–every individual
has
an
equal
it to be an SRS.
chance of being selected
–every set of n individuals has an
equal chance of being selected
Stratified
random sample
Homogeneous groups are groups
that are alike based upon some
the group
Supposecharacteristic
we were to oftake
a stratified
random sample members.
of 100 EHS students.
Since students are already divided by
grade level, grade level can be our
strata. Then randomly select 25
students from each grade level.
• population is divided
into homogeneous
groups called strata
• SRS’s are pulled from
each strata
Systematic
random sample
Suppose we want to do a systematic random
sample of EHS students - number a list of
students
(There are approximately 2000 students – if we
want a sample of 100, 2000/100 = 20)
• select
sample
by
Select a number between 1 and 20 at
random.
That student
will be the first
following
a
systematic
student chosen, then choose every 20
student from there.
approach
• randomly select where to
begin
th
Cluster Sample
Suppose we want to do a cluster sample of
EHS students. One way to do this would
be to randomly select 10 classrooms during
2nd period. Sample all students in those
rooms.
• based upon location
• randomly pick a
location & sample all
there
Multistage
sample
To use a multistage approach to sampling
EHS students, we could first divide 2nd
period classes by level (AP, Honors,
Advanced, etc.) and randomly select 4
second period classes from each group.
Then we could randomly select 5 students
from each of those classes. The selection
process is done in stages.
• select successively
smaller groups within
the population in stages
• SRS used at each stage
Bias
favors certain outcomes
Anything
that
causes
the
Is not the data
same
as
to be wrong! It
might
be
attributed
to
sample error
or
sample
the researchers, the
respondent,
or
to
the
variabilitysampling method.
Identify the sampling design
1)The Educational Testing Service
(ETS) needed a sample of colleges.
ETS first divided all colleges into
groups of similar types (small
public, small private, etc.) Then
they randomly selected 3 colleges
from each group.
Stratified random sample
Identify the sampling design
2) A county commissioner wants to
survey people in her district to
determine their opinions on a
particular law up for adoption. She
decides to randomly select blocks in
her district and then survey all who
live on those blocks.
Cluster sampling
Identify the sampling design
3) A local restaurant manager wants
to survey customers about the
service they receive. Each night
the manager randomly chooses a
number between 1 & 10. He then
gives a survey to that customer,
and to every 10th customer after
them, to fill it out before they
leave.
Systematic random sampling
Sources of
Bias
• things that can cause
bias in your sample
• cannot do anything
with bad data
Voluntary
response
• People
chose
to
respond
An example
would
be
the
surveys
Remember – the way toin
magazines that ask readers to mail in
determine
voluntary
•the
Usually
onlyexamples
people
survey.
Other
arewith
callin shows,response
American Idol,
is: etc.
very strong opinions
Remember, the respondent selects
respond
themselves
to participate in the
Self-selection.
survey.
Convenience
sampling
The data obtained by a convenience
sample will be biased – however this
method is often used for surveys &
results reported in newspapers and
An example would be stopping
magazines!
friendly-looking people in the mall to
survey. Another example is the
surveys left on tables at restaurants
- a convenient method!
•Ask people who are
easy to ask
•Produces biased
results
Undercoverage
People with unlisted
phone numbers – usually
high-income families
•some groups of
People without
phone numbers –
population
left
Suppose
you take a are
usually
lowsample by randomly
income families
out names
of the
selecting
from sampling
the phone book –
process
some
groups will not
People with ONLY cell
have the opportunity
of being selected.
phones, which is
becoming more common.
Nonresponse
Because of huge telemarketing
efforts in the past few years,
telephone surveys have a MAJOR
People
are
chosen
by
the
problem
with
nonresponse.
One way
to
help
with
theresearchers,
problem
BUT refuseistotoparticipate.
of nonresponse
make followup contact with people who are
NOT
self-selected!
not home
when
you first contact
them.
This is often confused with voluntary
response.
• occurs when an individual
chosen for the sample
can’t be contacted or
refuses to cooperate
• telephone surveys 70%
nonresponse
Response bias
Suppose we wanted to survey high
school students on drug abuse and
we used a uniformed police officer
to interview each student in our
sample – would we get honest
Response biasanswers?
occurs when for some
reason (interviewer’s or respondent’s
fault) you get incorrect answers.
• occurs when the
behavior of respondent
or interviewer causes
bias in the sample
• wrong answers
Wording
of
the
The level of vocabulary should be
appropriate
for the
you
Questions
mustpopulation
be worded
as
Questions
are surveying
neutrally
as possible to avoid
influencing
the influence
response.
• wording
can
the
– if surveying, say, young
children,
you should
answers that
arethen
given
avoid complex vocabulary.
• connotation of words
if surveying
doctors,
•– use
of “big”
words or
then use more complex,
technical
words
technical wording.
Source of Bias?
1) Before the presidential election of
1936, FDR against Republican ALF
Landon, the magazine Literary Digest
predicting Landon winning the election in
a 3-to-2 victory. A survey of 10 million
people. George– Gallup
surveyed
only
Undercoverage
(one possible
answer)
This
50,000
people
and predicted
that People
was
during
the Great
Depression.
Roosevelt
would win.
The Digest’s
surveyand
who
could afford
magazines,
telephones
came
magazine
cars
atfrom
the time
weresubscribers,
mostly fromcar
highowners,
telephone
directories,
etc.
income
homes
and thus
mostly Republican.
2) Suppose that you want to
estimate the total amount of
money spent by students on
textbooks each semester at
SMU.
You
collect
register
Convenience sampling – it takes little
effort to catch
at the
receipts
for students
students
as store
they
or
leave
the
bookstore
during
Undercoverage – students who buy
lunch
oneon-line
day. bookstores are not
books from
included.
3) To find the average
value of a home in River
Oaks, one averages the
price of homes that are
listed for sale with a
Undercoverage – leaves out homes
realtor.
that are not for sale or homes that
are listed with different realtors.
(other answers are possible)