Population, sampling frame and observed sample
The people you want to describe may differ from the people you could contact and the people who answered.
The target population is everyone the conclusion should describe. The sampling frame is the list from which participants can be reached or sampled. The observed sample is who is actually recorded. For a platform survey these could be all enrolled students, students with active accounts, and those who reply.
Coverage error arises when the frame excludes parts of the target population. Nonresponse occurs when included or invited people do not provide an answer. A larger convenience sample does not repair systematic exclusion. Random sampling is a selection design, not a synonym for "a lot of data."
The Week 1 slides assign additional sampling strategies as textbook reading, but the textbook is not attached. Know the basic contrast: a probability sample uses a known selection mechanism; convenience selection is driven by access. More advanced sampling designs should be checked against your assigned reading if they are in scope.
Sources: W1 PDF pp. 3, 13-17, 22-30, 32-53, 57-60; L1 Parts 1-5.
The lecture connects nonresponse with representation. The survey separates group invitation shares from reply shares and asks what MCAR/MAR/MNAR mean.
Practice problems
Supplementary study questions on the lecture concepts — not verbatim exam questions. Each answer counts toward spaced review.
1. Conceptual understanding
Definitions and distinctions.
Everyone enrolled is the target, but only active accounts are invited. What risk appears?
Will a huge convenience sample necessarily fix selection bias?
2. Application
Apply the concept to a case or calculation.
All 500 students are invited and 300 reply. Who is the observed sample?
A student is invited but provides no rating. What is this?
3. Interpretation and limitations
Evaluate assumptions and justify conclusions.
What makes selection a probability sample?
A survey reaches only students attending an optional club. What limits a university-wide claim?
Explain it in writing
Self-checked, not marked. Write a few sentences, then compare with the example answer. Your writing is saved as you type.
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Coverage error means the contact or selection list leaves out part of the target population. Nonresponse means a person was included or invited but did not provide the measurement.
Check: did you answer the question, use evidence or working, and avoid claiming more than the evidence supports?
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People with particular experiences may be more likely to reply. A large sample can still overrepresent them. More records do not automatically remove systematic selection.
Check: did you answer the question, use evidence or working, and avoid claiming more than the evidence supports?