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SAMPLING HARD-TO-REACH POPULATIONS
Karen Foote Retzer
February 18, 2015
www.srl.uic.edu
GENERAL INFORMATION
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• Please hold questions until the end of the presentation
• Slides available at http://www.srl.uic.edu/SEMINARS/Spring15Seminars.htm
• Please raise your hand so that I can see that you can hear me
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OUTLINE
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• Introduction
• Sampling Hard-to-Reach Populations using Probability Sampling
• Non-Probability Sample Designs
HARD-TO-SURVEY POPULATIONS
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• Hard to sample
• Hard to identify
• Hard to find
• Hard to persuade
• Hard to interview
(Tourangeau, 2014)
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EXAMPLES
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� Homeless
� Prostitutes
� Migrants
� Illegal Immigrants
� Rare racial/ethnic groups
� Visitors to a museum
� Travelers through airports
� Gay men
� Smokers
PROBABILITY VS. NON-PROBABILITY
SAMPLING
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Probability
• Generalize to the population from which sample is drawn
• Unbiased results
• Known, non-zero probability of selection
Non-Probability
• Exploratory research
• Convenience
• Probability of selection unknown
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TRADITIONAL PROBABILITY SAMPLES
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• Screen all cases to locate eligible respondents
• Disproportionate Stratified Sampling
• Dual Frames
PROBABILITY SAMPLE: SCREEN ALL
CASES TO FIND ELIGIBLE RESPONDENTS
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Target population: adults who smoke cigarettes.
Sample and screen individuals in randomly selected households.
� Area probability (face-to-face)
• Address-based sample (mail)
Time consuming & expensive.
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PROBABILITY SAMPLE: DISPROPORTIONATE
STRATIFIED SAMPLING
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Target Population: Households with income $200,000 or higher
Use census data to find blockgroups with households having higher incomes.
Sample households from all blockgroups, but oversample those with higher incomes.
Post-stratification weights.
PROBABILITY SAMPLE:
DUAL/MULTIPLE FRAMES
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Target population: households with pets
Use two frames:
Households
Lists of people from veterinarian offices
Must determine how to weight the final dataset.
Dual frames becoming more common with
changes in telephone technology.
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DUAL FRAME: PHONE SURVEY
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Contacting a nationwide
sample by phone
PROBABILITY SAMPLES: DUAL LANDLINE
AND CELL FRAMES
• Increase of general population that is cell only
• Cell phone usage differences within the population
• Race and ethnicity
• Age
• Income
• Region of country
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PROBABILITY SAMPLE: DUAL LANDLINE
AND CELL FRAMES
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Contacting national sample using
cell phone and landline phones
(dual frame design)
PROBABILITY SAMPLE: DUAL LANDLINE
AND CELL FRAMES
• Difficult to target smaller geographies by cell phone
• Cell-phone-only individuals who have a cell number from a state, but who do not live in the state.
• Cell-phone-only individuals who live in state who do not have cell number from that state
• Smaller geographies even more difficult to target.
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RECAP: PROBABILITY SAMPLE
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Each element has a known, non-zero probability of selection
Results can be generalized to the population from which the sample was drawn.
• Screening
• Disproportionate Sampling
• Dual frame
LESS TRADITIONAL
SAMPLE DESIGNS
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• Location (venue-based)
• Time location
• Respondent-driven
• Snowball
• Network/multiplicity
• Convenience
• Surnames
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LOCATION SAMPLING
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� Location sampling can take place at . . .
� Museums
� National Parks
� Shelters
� Street corners
� Two types of sampling
� Sampling visits
� Sampling visitors
TIME LOCATION SAMPLING
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An extension of location sampling where
the timing of the sample selection is also
taken into consideration.
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SNOWBALL
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� Initial sample of respondents selected
� Convenience
� Randomly
� Respondents identify others to be interviewed
RESPONDENT-DRIVEN SAMPLING
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� Initial sample of respondents selected (similar to snowball)
� Respondents become the ‘seeds’ and are given coupons to give to others in their networks who are eligible
� Maintains privacy for those recruited
� Respondents receive monetary incentive
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MULTIPLICITY SAMPLING
• Includes non-household members with whom informant is familiar, such as close relatives
• Informant must know needed information about network members
• Informant must know size of network
• Weighting
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SURNAME LISTS
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� Some populations can be identified by surname
� Hispanics
� Chinese
� Korean
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CONVENIENCE
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� Questionnaires that potential respondents can pick up at given location
� Persons who take part in studies recruited from flyers
� Hotel guests who complete questionnaires in hotels
� Respondents who are intercepted at malls, etc.
HOW TO SELECT SAMPLE DESIGN
It depends on . . .
• How common or rare your population is
• How geographically clustered/dispersed your population is
• Whether or not you want to generalize your findings
• Why the respondents are hard to reach
• Whether or not you have a list
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SELECTED BIBLIOGRAPHY
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Blumberg SJ, Luke JV. Wireless substitution: Early release of estimates from the National Health Interview Survey. July-December 2012. National Center for Health Statistics. July 2013. Available from: http://www.cdc.gov/nchs/nhis.htm.
Tourangeau, R., Edwards, B., Johnson, T., Wolter, K., & Bates, N. (Eds.). (2014). Hard-to-survey populations. Cambridge: Cambridge University Press.
Wagner, J., & Lee, S. (2015). Sampling Rare Populations. In T. Johnson (Ed.), Handbook of Health Survey Methods
(pp. 77-104). Hoboken: John Wiley & Sons, Inc.
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