Survey Sampling
Contents
Sample Frames
Common frames used for survey sampling are:
Census Bureau surveys are excellent sources of regional, hierarchical data (i.e. states > counties > tracts)
- U.S. Postal Service Delivery Sequence files
- Random digit dialing (RDD)
Sample Type
Propability sampling
All members of a population have a non-zero chance to be contacted in a survey instrument. Traditional statistics rely on this assumption.
Examples:
- Administrative surveys
- Surveys with random recruitment (as by random digit dialing)
Non-probability sampling
Some members of a population are certain to be contacted or not be contacted.
Examples:
- Panel surveys
- River surveys (i.e. surveys with open recruitment, as by banner ads)
Survey Allocation
Allocation is the distribution of sample size across domains.
Designing Domains
The key considerations are:
- are some splits more important to others?
- if studying military recruitment, then sex/gender is a strong split
- what splits will be used for reporting?
- what is the expected response rate?
- if too few responses are expected from a domain, then splits should be reconsidered
- what is the desired margin of error?
Stratified Allocation
Stratification is the process of dividing the population into discrete stratum, and then sampling from the strata.
Equal allocation is taking the same number from each stratum.
Proportional allocation is taking a number proportional to the size of that stratum.
Neyman allocation is an optimization of a key measure's margin of error against cost. It assumes a fixed cost per contact.
On the other hand, if cost is assumed variable, it becomes an optimal allocation.
Note that, if all measures are equally varied, proportional allocation is essentially the same as a Neyman allocation.
Sampling Methods
Simple Random Sampling (SRS) is essentially sorting randomly and taking the first N cases.
Stratified Random Sampling (STSRS) is the above process applied to a stratified sample, using proportional allocation.
Systematic sampling is any form of sampling that takes every Nth case from a list. The key is then how the list is ordered.
Probability Proportionate to Size (PPS) ensures that chance to be contacted increases with the magnitude of some measure. For example, in a study of utility customers, the largest consumers of that utility should almost always be contacted.
Multi-Stage Methods
Randomly select primary sampling units (PSU) like census tracts, then randomly select the actual targets (i.e. households) as secondary sampling units (SSU).
Cluster sampling is a two-stage method where all members of the sampled PSUs are contacted.
Common in face-to-face interviews, due to extraordinary costs of that mode.
Multi-Phase Methods
Sample for a screener, then re-sample based on the information collected in the screener. In most cases, all responses from the target group are re-contacted, while a random sample of others are re-contacted.