Basic sampling issues
14 important questions on Basic sampling issues
Is it true that a census provides more accurate results than a sample?
No not necessarily true, there are many impediments to actually obtaining information from every member of the population, so the ideal census is seldom attainable.
How to specify a population of interest?
In terms of geographic area, demographics, product usage characteristics, awareness measures.
Also include security questions to exclude people from the survey.
Is there often a perfect correspondence between the sampling frame and the population of interest?
No, seldom, because the list of populaton members is often not complete and accurate.
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Two types of sampling methods
- Probability sampling: every element of the population has a known nonzero likelihood of selection.
- Nonprobability sampling: samples in which specific elements from the population have been selected in a non-random manner.
Disadvantages of probability sampling over nonprobability sampling
- Usually more expensive
- Rules for selection increase interviewing costs and professional time spent in designing and executing the sample design.
How to determine the sample size for a nonprobability sample
Rely on factors such as available budget, rule of thumb and number of subgroups to be analyzed.
How to determine the sample size for a probability sample
Use formuleas to calculate the sample size required, given the target levels of acceptable error (difference between sample result and population value) and levels of confidence (likelihood the confidence interval will include the population value).
Accuracy of sample results is affected by two types of error
- Sampling error
- Measurement error
When is there no difference between disproportional and proportional allocation?
When the distributions of the characteristics under consideration (stratums) have the same standard deviation.
Difference of cluster sampling and stratified sampling
Stratified sampling: select elements from each subgroup (stratum).
Cluster sampling: select all elements in the subgroup (one stage) or from a sample of the elements in the subgroup (two-stage).
Cluster samples have ... cost efficiency and ... statistical efficiency
Greater cost efficiency. Lower statistical efficiency due to larger sampling error than simple random sample or stratified sample of the same size.
Two key differences between quota samples and stratified samples
- Respondents for stratified samples are selected randomly, for quota samples they are not.
- Classification factors used for stratified samples are selected based on the existence of a correlation between the factor and the behaviour of interest.
Advantages of internet interviewing
Target respondents can complete the survey at their own convenience
Data collection is relatively inexpensive
Interview can be administered under software control
Survey can be completed quickly
Disadvantage of internet interviewing
People in cyberspace do not correctly represent the general population, as they tend to be more whith, richer, male and tech savvy.
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