Sampling
Errors
› Random sampling error –
expressed as the margin
of error.
Sampling
Errors
› Bad sampling methods –
voluntary response samples,
convenience samples.
Sampling
Errors
› Under coverage – happens when
some groups in the population are
not included in the sampling frame.
(A sampling frame is a list of
individuals from which a sample is
drawn.)
› What happens if under coverage
occurs???
Non-Sampling
Errors
Processing errors –
mechanical errors,
arithmetic errors, data
entry errors
Non-Sampling
Errors
Response errors – person
lies about answer,
remembers details
incorrectly, doesn’t
understand question
Non-Sampling
Errors
 Non-response errors – failure to obtain data
from individual (refuses to cooperate or
cannot be contacted)
This is becoming the biggest error
category, which produces the most bias…
(people don’t want to answer questions over
the telephone; caller ID and answering
machines prevent contact; and the elderly
are often reluctant to participate)
 Non-sampling
errors
› The wording of questions – if the
question asks if the subject
“favors” some policy as a
means to an end, it is a loaded
question that draws positive
responses from subjects who are
worried.
› Example: “Do you favor imposing
a closed campus for Irvington
students if it means increased
safety for your children?”
› Example: “Do you favor leaving
Irvington campus open for students
during lunch time so that nearby
businesses would not need to close
due to lack of customers?”

Stratified Random Sample
› Divide the sampling frame into distinct
groups (strata).
› Take SRS’s of each strata.
› Example: Using the school’s population, the
sampling frame can be divided into
freshmen, sophomores, juniors, seniors, and
staff (5 strata).
Cluster Sample
 Can be used when the population falls into
naturally occurring subgroups (like zip
codes).
 To select a cluster sample, divide the
population into groups (clusters) and select
all the members in one or more (but not all)
the clusters.
 Other examples: different periods of the
same course, different branches of a bank,
etc.





Systematic Sample
Each member of the population is assigned a
number. The starting number is randomly
selected, and then the sample members are
selected at random intervals from the starting
member.
Example: randomly chose the 15th person,
then the interval was every 4th person after
that, so the 15th, 19th, 23rd, 27th, and so on, were
chosen for the survey.
Be careful using this in case there are regularly
occurring patterns already in the population
(every 4th person has a common quality).
Who
carried out the survey?
What was the population?
How was the sample selected?
How large was the sample?
What
was the response rate?
How were the subjects contacted?
When
was the survey conducted?
What were the exact questions?
 Government
statistical offices answer these
questions when they announce results;
national opinion polls don’t always release
the response rate (usually low). Newscasters
usually leave out a lot of this information
(boring); interest groups/newspapers don’t
give this info because their methods are
usually unreliable.
 If someone doesn’t give all the information
stated above, be skeptical!!!
Whatever!
 NEW
YORK, Oct. 7 (UPI) -- Nearly half of Americans
surveyed for a new poll said "whatever" was the most
annoying word that could be used in a conversation.
 The Marist Poll received votes for "You know" and
"anyway," but they weren't even close to the 47
percent that pegged "whatever" as something that
had definitely run its course.
 The breakdown found "whatever" to be particularly
tiresome among respondents in the Midwest, Latinos
and people under 45 years old.
 Phrases that bug Americans include "it is what it is" and
"at the end of the day."
 The poll, based on 938 responses, had a margin of
error of 3.5 percent.
Page 110…114, #2.58-2.60, 2.62-2.66
 Bring a Gallup Poll for Friday’s group
activity


Friday’s homework…
Page 119…122, #2.68-2.71, 2.74