COMM 301:
Empirical Research in
Communication
Kwan M Lee
Lect6_1
1
Sampling
Non-probability sampling
• Things to know by the end of the lecture
– How is non-probability sampling different from
probability sampling
– Why and when do we use non-probability
sampling?
– What are the non-probability sampling
techniques?
2
Sampling
Non-probability sampling
• Non-probability sampling
– do not rely on mathematical rules to select the sample
– cannot estimate sampling error
• So why do we use non-probability sampling?
– Sometimes, using probability sampling cannot get a meaningful
sample (illustrated below)
– Sometimes, it costs less effort, time, resources
• Non-probability can supplement probability sampling
• Cf. What is the main advantage of probability sampling
over non-probability sampling?
– Calculation of sampling errors  increase generalizability
3
Sampling
Non-probability sampling
• Non-probability sampling methods
– convenience
– volunteer
– purposive
– quota
– network
4
Sampling
Non-probability sampling
• Convenience sampling
– select elements because they are accessible
– no use of mathematical rules
– e.g. surveys conducted in the mall
5
Sampling
Non-probability sampling
• Volunteer sampling
– select based on agreeing to participate
– for human subjects, especially minors, this is
a key ethical principle
– are volunteers different from non-volunteers?
6
Sampling
Non-probability sampling
• Purposive sampling
– Key advantage of purposive sampling over
probability sampling
• Make it possible to select samples with very rare
and unique characteristics (which we plan to
study).
– e.g. study CEO communication in
bankruptcies of large firms
• Montgomery Wards, K-Mart, Enron
– common in case studies
7
Sampling
Non-probability sampling
• Quota sampling
– similar to stratified sampling
– but without use of mathematics-based
random selection
– use of non-probability methods (convenience,
volunteer, etc.)
8
Sampling
Non-probability sampling
• Network sampling
– select members based on other sample
members identifying
– used in qualitative research
– e.g. in-depth interviews in organizations
9
Sampling
Non-probability sampling
• In general, when using non-probability
sampling, need to consider carefully
issues of representativeness
10