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Introduction to
Sample Surveys
政府統計處
Census and Statistics Department
Sample Surveys
 Sample survey has widespread applications,
ranging from surveys conducted for business
purposes to collection of data for use in public
affairs and social studies.
 In order to ensure the reliability of survey
results, it is essential that great care in
addition to adequate professional knowledge
be applied in the planning and conduct of
surveys and in the analysis of data.
政府統計處
Census and Statistics Department
Major Steps in Conducting
a Sample Survey





Overall planning
Design and selection of sample
Design of questionnaire
Collection of raw data
Compilation and analysis of statistics
and dissemination of survey results
政府統計處
Census and Statistics Department
Overall Planning
 Clearly defined survey objectives
 Clearly defined target population
 Clearly defined statistical standards
(definition of terms, counting rules,
classification, statistical methodologies)
 Respondents should be able to provide
the information required.
 Adequate resources such as manpower
and time must be available.
政府統計處
Census and Statistics Department
Design and Selection of Sample
 Sampling deals with the selection of a
number of elements, i.e. a sample.
 A sample can be drawn using different
methods. A probability sample, with
each element having a known, non-zero
chance of being included, should be
used as far as practicable so that
scientific inference can be drawn from
the survey results.
政府統計處
Census and Statistics Department
Design and Selection of Sample
 Non-probability sampling methods, such
as haphazard sampling, should be
avoided. Such methods are not scientific
and bias usually exists in survey results.
 A complete and up-to-date sampling
frame should be acquired.
政府統計處
Census and Statistics Department
Design and Selection of Sample
 In case of unequal probability of
selection, it is necessary to ensure that
proper weighting methods are applied to
survey results.
 Using established statistical
methodology to compute the required
sample size.
 Once the sampling units are selected,
alterations are not allowed.
政府統計處
Census and Statistics Department
Common Probability
Sampling Methods





Simple random sampling
Systematic sampling
Stratified sampling
Clustering sampling
Multi-stage sampling
政府統計處
Census and Statistics Department
Non-probability Sampling Methods
 Selection of elements is based on
subjective judgment and experience, but
not a random manner
 Common non-probability sampling
methods
Street interview/mall intercept
Quota sampling
Respondent-initiated telephone polling
Internet survey
政府統計處
Census and Statistics Department
Non-Probability Sampling Methods
 Why unreliable?
Scientific inference cannot be drawn from
the sample data (results are confined to
describing the group of respondents, but cannot be
extracted to the entire population)
Level of precision of the estimates cannot be
scientifically assessed
Biases will most likely exist in the survey
results
政府統計處
Census and Statistics Department
Non-Probability Sampling Methods
 Why unreliable (cont’d)?
No way to know how exactly the group of
respondents is formed
• In street interviews, the interviewers tend to
select those “friendly faces” for interview.
• In self-selecting polls, only those who have
strong views on the survey may volunteer to
participate.
The results may be inaccurate and
misleading, hence not of much use.
政府統計處
Census and Statistics Department
General Principles in
Designing Questionnaires
 Questions should be relevant to the
survey objectives
 Questions should be arranged in a
proper order
 Use screening question to enhance the
flow of the questions.
 Use an appropriate language
 Clear instructions
政府統計處
Census and Statistics Department
General Principles in
Designing Questionnaires
 Questions wordings should be
appropriate, specific and precise
 Avoid leading questions or questions
being loaded in favour of a particular
response
 Avoid difficult vocabulary
 Avoid composite and double negative
questions
 Beware of memory error
政府統計處
Census and Statistics Department
General Principles in
Designing Questionnaires
 “Don’t know/No opinion” should be
included as appropriate
 Long questionnaires are undesirable.
 Questionnaires should be tested on
some prospective respondents before
finalized.
政府統計處
Census and Statistics Department
Collection of Raw Data
 Methods of data collection for surveys
include:
Self-administered questionnaires by mail
Personal interviews
Telephone interviews
Computer Assisted Telephone Interviewing
(CATI)
 Very often, mixed modes of data
collection can be used.
政府統計處
Census and Statistics Department
Collection of Raw Data
 An appropriate mode should be selected
by carefully considering respondents'
willingness to co-operate, the degree of
complexity of the subject of enquiry and
other relevant factors (e.g. practicability
of using personal or telephone
interviews).
政府統計處
Census and Statistics Department
Comparison of Different Data
Collection Methods
Characteristics
Personal
interview
High
High
High
Telephone
interview
Medium
Medium
Medium
Mail
questionnaire
Low
Low
Low
Non-response/
non-contact rate
Suitability for scattered
population
Interviewer bias
Relatively
lower
Costly
Medium to
high
Less costly
High
Medium
Yes
Yes
No
Response quality
Length of questionnaire
High
May be
longer
May be more
complicated
Not suitable
Medium
Preferably
short
Simple
Low
Preferably
short
Simple
Easier
Easier
Cost
Manpower
Time consuming
Design of questionnaire
Asking embarrassing
questions
Collection of Raw Data
 Interviewers should be trained before
they start working and closely
supervised during fieldwork to ensure
their quality of work.
 Identity and information supplied by
individual respondents should be kept
confidential. The survey results are to be
presented in the form of aggregate
statistics.
政府統計處
Census and Statistics Department
Collection of Raw Data
 Every effort should be made to achieve a
high response rate (or reducing the
number of non-responses).
 Methods to reduce non-responses:
Keep the questionnaire brief and concise
Consider rewards to respondents
Better publicity measures (e.g.
advertisement, advance letters)
Assurance of confidentiality of individual
data
政府統計處
Census and Statistics Department
Collection of Raw Data
 Methods to reduce non-responses (Cont’d):
More experienced interviewers, especially
when handling refusals
Visit households in evening time
Visit at different time of different days
Increase the number of re-visits/call backs
Use self-administered questionnaires in case
of non-contact
政府統計處
Census and Statistics Department
Compilation and Analysis of Statistics
 Data should be carefully and thoroughly
checked before compilation .
 Appropriate statistical methodology
should be adopted in compiling and
analysing data.
政府統計處
Census and Statistics Department
Reporting and Assessing
the Reliability of Surveys
 To enable readers to make judgment on
whether the findings are credible, a good
survey report should include:
Sponsorship of the survey
Population covered
Sampling method
Mode of data collection
Time period of data collection
Wording of questions
政府統計處
Census and Statistics Department
Reporting and Assessing
the Reliability of Surveys
 A good survey report should include
(cont’d):
Sample size and response rate
Point estimates and confidence intervals (if
possible)
 Likely sources of non-sampling errors
 Information supplied by individual
respondents should not be disclosed.
政府統計處
Census and Statistics Department
Reporting and Assessing
the Reliability of Surveys
 One may access the reliability of a
survey by asking the following
questions :
Has probability sampling methods been
used ?
Is the sample size reasonably large ?
Is the questionnaire design proper ? Any
leading questions or wordings ?
政府統計處
Census and Statistics Department
Reporting and Assessing
the Reliability of Surveys
 One may access the reliability of a
survey by asking the following questions
(cont’d):
What is the interviewing method ? Any
improper influence by interviewers or third
parties during the interview ?
Is the response rate too low ?
Are the sampling errors of acceptable
magnitude ? How about non-sampling
errors and biases ?
政府統計處
Census and Statistics Department