1. Non-sampling Errors in Polls and Surveys
Dru Rose

2. Methods of Data Collection
Surveys
and Polls
Experimental
Studies
Observational
Studies
Dru Rose

3. Polls and Surveys Poll Survey Few questions Multi-choice type only
Many questions
May have branches and skips
May have a “tick multiple boxes” option
May have open-ended, write a comment questions
Dru Rose

4. Sampling Sampling Errors Non-sampling (random process) Errors
Selection bias Non-response bias
Self selection bias Question effects
Behavioural considerations
Interviewer effects Survey-format effects
Transfer of findings

5. Not eligible for survey
Selection Bias: Population sampled is not exactly the population of interest.
Target population (e.g. adults in NZ)
Sampling frame (e.g. households with a landline phone)
Not included in sampling frame
Cannot be contacted
SAMPLED POPULATION
Not eligible for survey
Refuse to respond
Incapable of responding

6. Sources of Non-sampling Errors
Non-response bias
When people who have been targeted to be surveyed do not respond:
Potential bias if non-respondents are likley to behave differently to respondents with respect to the question being asked.
e.g. Non-respondents in an employment survey are likely to be those who work long hours.

7. Sources of Non-sampling Errors
Self-selection bias
People decide themselves whether to be surveyed or not.
Dru Rose

8. Self-selection bias: phone-in or internet polls

9. Sources of Non-sampling Errors
Question effects
Subtle variations in wording can have an effect on responses.
e.g. “Should euthanasia be legal?”
vs. “Should voluntary euthanasia be legal?”
People are more likely to favour “voluntary” euthanasia.

10. 18 August 1980 New York Times/CBS News Poll
“Do you think there should be an amendment to the constitution prohibiting abortions?”
Yes 29% No 62%
Later the same people were asked:
“Do you think there should be an amendment to the constitution protecting the life of the unborn child?”
Yes 50% No 39%

11. Question Effects in the NZ Census
1986: “What is your ethnic origin?
(Tick the box or boxes which apply to you.)
1991: “Which ethnic group do you belong to?” (Tick the box or boxes which apply to you.)
1996: “Tick as many circles as you need to show which ethnic group(s) you belong to.”
Changes in wording in the Question from 1986 to 1996 makes comparisons unreliable-apparent change in dual ethnicity may be due to wording.
Ethnicity
1986
1991
1996
Single
94.6
94.3
81.0
European
81.2
78.1
65.8
Maori
9.1
9.6
7.6
Two Ethnicities
4.0
4.5
11.2
European & Maori
2.9
2.7
4.7
Two European gps
0.0
0.6

12. Sources of Non-sampling Errors
Behavioural considerations
People tend to answer questions in a way they consider to be socially desirable.
e.g. pregnant women being asked about their drinking habits may be reluctant to admit that they drink alcohol
Dru Rose

13. Sources of Non-sampling Errors
Interviewer effects
Different interviewers asking the same question can obtain different results.
e.g. the sex, race, religion , manner
of the interviewer
may influence how people respond to a particular question.
Dru Rose

14. Interviewer Effects in Racial Questions
In 1968, one year after a major racial disturbance in Detroit, a sample of black residents were asked:
“Do you personally feel that you trust most white people, some white people or none at all?”
White interviewer:
35% answered “most”
Black interviewer:
7% answered “most”

15. Sources of Non-sampling Errors
Survey-format effects
-question order e.g.
“To what extent do you think teenagers are affected by peer pressure when drinking alcohol ?”
followed by:
“ Name the top 5 peer pressures you think teenagers face today.”
-survey layout
-interviewed by phone or in-person
or mail.

16. Sources of Non-sampling Errors
Transferring findings
Taking the data from one population and transferring the results to another.
e.g. Auckland opinions may not be a good indication of New Zealand opinions.
sample
Auckland
New Zealand

17. Non-sampling Errors can be much larger than sampling errors
are always present
can be virtually impossible to correct for after the completion of survey
virtually impossible to determine how badly they will affect the result
good surveys try to minimize them in the design of the survey (e.g. do a pilot survey first)

18. Surveys / Polls A report on a sample survey/poll should include:
Who carried it out and who funded it
target population (population of interest)
sample selection method
the sample size and the margin of error
the date of the survey
the exact question(s) being asked
the results
the claims (inferences) made