Line charts: time order, gaps and windows
A line joins observations in time order. Missing weeks should appear as gaps, and the chosen time window must be stated because it can change the story.
A line connects observations in time order; equal horizontal steps should mean equal time intervals (W6 p. 17). For a fixed cohort of 10 students, week 1 has 50 logins (50÷10 = 5 per student) and week 6 has 42 (42÷10 = 4.2). The change is 4.2-5 = -0.8 logins per student, a general decline. Dividing by a fixed cohort keeps the denominator constant; if the cohort changed, totals would mix activity with enrolment.
Missing is not zero (W6 p. 18). If week 4 was not recorded, leave a gap between the observed segments. Plotting 0 claims an observed collapse that never happened; silently joining weeks 3 and 5 invents a value. A gap means "unobserved"; a zero means an observed value of zero. (The slide's figure also contains a stray straight line from week 1 to week 6, a drawing artefact; the intended display is two observed segments separated by a gap.)
The time window changes the comparison (W6 p. 28). With a fixed cohort and the same 0-10 scale:
- Whole period, weeks 1-8: (6-8)/8×100 = -25%.
- Recent period, weeks 6-8: (6-5.3)/5.3×100 ≈ +13.2%.
Both are true. A headline such as "activity is recovering" or "activity fell by a quarter" needs its dates, and cherry-picking a window to support a preferred story is misleading.
Source: W6 PDF pp. 1-37. Most numbers in the lecture are illustrative or constructed for teaching; pp. 26-27 reuse the Week 5 synthetic registration counts. The Week 6 lab manual (L6) practises Week 5 EDA reasoning and is worked through section by section in Lab work.
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.
Week 4 was not recorded. How should the line chart show it?
A fixed cohort's mean logins go from 8 (week 1) to 6 (week 8). What is the relative change?
2. Application
Apply the concept to a case or calculation.
A fixed cohort of 10 logs 42 logins in week 6. What is the mean per student?
Why does the lecture use a fixed cohort for weekly means?
3. Interpretation and limitations
Evaluate assumptions and justify conclusions.
Mean logins go from 5.3 to 6 over weeks 6–8. What is the relative change?
What should equal horizontal steps on a time axis represent?
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
A 0 would claim an observed value of zero logins, an apparent collapse. In fact the value is unknown, so the line should show a gap.
Check: did you answer the question, use evidence or working, and avoid claiming more than the evidence supports?
Show an example answer
Name the windows, for example “Down 25% from week 1 to week 8, though up about 13% since week 6.” Both statements are correct for their stated periods.
Check: did you answer the question, use evidence or working, and avoid claiming more than the evidence supports?