census and statistics department introduction to sample surveys
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政府統計處Census and Statistics Department
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 Conductinga 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 ProbabilitySampling 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 methodsStreet interview/mall interceptQuota samplingRespondent-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 th
e 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 res
pondents is formed• In street interviews, the interviewers tend to sele
ct 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 que
stions 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 mailPersonal interviewsTelephone interviewsComputer 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).
Comparison of Different Data Collection Methods
Characteristics Personal interview
Telephone interview
Mail questionnaire
Cost High Medium LowManpower High Medium Low
Time consuming High Medium Low
Non-response/non-contact rate
Relatively lower
Medium to high
High
Suitability for scattered population
Costly Less costly M edium
Interviewer bias Yes Yes No
Response quality High Medium Low
Length of questionnaire May be longer
Preferably short
Preferably short
Design of questionnaire May be more complicated
Simple Simple
Asking embarrassing questions
Not suitable Easier Easier
政府統計處Census and Statistics Department
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 conciseConsider rewards to respondentsBetter 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 refusalsVisit households in evening timeVisit at different time of different daysIncrease the number of re-visits/call backsUse 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 Assessingthe Reliability of Surveys
To enable readers to make judgment on whether the findings are credible, a good survey report should include:Sponsorship of the surveyPopulation coveredSampling methodMode of data collection Time period of data collectionWording of questions
政府統計處Census and Statistics Department
Reporting and Assessingthe 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 Assessingthe Reliability of Surveys
One may access the reliability of a survey by asking the following questions :Has probability sampling methods been use
d ?Is the sample size reasonably large ?Is the questionnaire design proper ? Any lea
ding questions or wordings ?
政府統計處Census and Statistics Department
Reporting and Assessingthe Reliability of Surveys
One may access the reliability of a survey by asking the following questions (cont’d):What is the interviewing method ? Any impro
per influence by interviewers or third parties during the interview ?
Is the response rate too low ?Are the sampling errors of acceptable magnit
ude ? How about non-sampling errors and biases ?
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