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Bob Foster wrote:
> All of these surveys violate the random sample assumption (in spades).
Principles of Survey Research
Part 5: Populations and Samples
ACM SIGSOFT Software Engineering Notes Volume 27 No 5, page 19
Probabilistic Sampling Methods
"Cluster-based sampling is the term given to surveying individuals
that belong to defined groups. For example, we may want to survey
all members of a family group, or all patients at specific hospitals.
Randomization procedures are based on the cluster, not the
individual. We would expect members of each cluster to give more
similar answers than we would expect from members of different
clusters. That is, answers are expected to be correlated within a
cluster. There are well-defined methods for analyzing cluster data,
but the analysis is more complex than that of a simple random
sample. (For an example, see .)"
Non-Probabilistic Sampling Methods
Convenience sampling involves obtaining responses from those
people who are available and willing to take part. The main
problem with this approach is that the people who are willing to
participate may differ in important ways from those who are not
willing. We see this kind of sampling particularly on web sites ...