Zero, missing and not applicable
These three meanings are different. Replacing them all with zero changes the question your numbers answer.
Zero lab-use hours means an enrolled student's observed use was zero. Missing hours means use is unknown. Not enrolled means the lab-use question may not apply; those students are outside a target population defined as enrolled lab students. Keep enrolled students with missing hours in the population count and report how many measurements are unavailable.
A missing marker may be an empty string, NaN, NA, a documented sentinel or an explicit not-applicable code. They are not automatically interchangeable. For example, the earlier lab's eq('') detects exact blank strings, not every possible kind of missing data in a general export.
Source: W3 PDF pp. 4-55. The 14-row registry example below comes from the Week 3 lab code you previously pasted in this conversation; it is separate from the lecture's smaller examples.
The lecture's summary context becomes a denominator check: all student rows for a response rate, recorded ratings for an observed mean.
The six quality dimensions are applied to a concrete task. The dictionary decides the score scale, date order and allowed programme meanings.
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.
What does an observed 0 hours of lab use mean?
When describing hours for enrolled lab students, who belongs in the population?
2. Application
Apply the concept to a case or calculation.
An enrolled student has verified 0 lab hours. Should this value enter an observed-hours mean?
A blank hours field tells us what amount of use?
3. Interpretation and limitations
Evaluate assumptions and justify conclusions.
A person is not enrolled in the lab. For a summary of enrolled students' hours, they are what?
Of 30 enrolled students, 24 have recorded hours, including zeros. What is valid n for the observed mean?
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.
Show an example answer
Yes. They belong in the enrolled population but not in the valid-measurement denominator for an observed mean. Report the missing count.
Check: did you answer the question, use evidence or working, and avoid claiming more than the evidence supports?
Show an example answer
It turns unknown contact details into a false stored value. Phone numbers are identifiers, not quantities where zero means no use. Keep an appropriate missing marker.
Check: did you answer the question, use evidence or working, and avoid claiming more than the evidence supports?