Match plots to variables
The chart should help answer the question and preserve the meaning of the variables. Always label units and counts.
| Variables and question | Useful view and what to inspect |
|---|---|
| One category | Bar chart or count/proportion table; name denominator |
| One numeric variable | Histogram; centre, spread, tails, peaks, gaps and flags |
| Numeric variable by category | Side-by-side boxplots or distributions with group sizes |
| Two numeric variables | Scatter plot; direction, form, strength, clusters and unusual points |
| Two categories | Cross-tabulation or conditional percentages with explicit denominators |
| A quantity over time | Ordered time plot; use the relevant time unit and watch missing periods |
In the histogram mini-example [42,47,53,58,61,64,65,78], bins 40-49,50-59,60-69,70-79 have counts 2,2,3,1. Counts add to 8. In code, define interval boundaries unambiguously so a boundary value is counted once, and ensure the final bin includes the permitted maximum.
A scatter plot can suggest a positive association while individual outcomes still vary widely. A selected subset of 50 plotted points is not the same evidence as a full-cohort correlation computed on 1,200 students. Label the plotted sample and the statistic's sample separately.
Source: W5 PDF pp. 3-51, 53. The lecture uses both a constructed 200-student support example and a synthetic 4,665-registration course dataset. They are different tables with different units.
Mean, median and skewness are clues; the four plots show what they miss. Changing bins changes the picture, not the data.
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.
Which plot directly examines the relationship between two numeric variables?
For five satisfaction levels, what is a clear first display?
2. Application
Apply the concept to a case or calculation.
Which plot helps inspect the shape of recorded minutes?
Which chart fits counts by programme?
3. Interpretation and limitations
Evaluate assumptions and justify conclusions.
How should a daily activity series be displayed to inspect time patterns?
A chart plots 50 selected points but a correlation uses 1,200 cases. What should be stated?
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
Each permitted value should belong to exactly one bin, including boundary values and the maximum. Counts should add to the number of values plotted.
Check: did you answer the question, use evidence or working, and avoid claiming more than the evidence supports?
Show an example answer
Similar-looking boxes can come from very different sample sizes. The counts help the reader judge how much evidence supports each group's pattern.
Check: did you answer the question, use evidence or working, and avoid claiming more than the evidence supports?