Module 4 — Graphs
Question: what does the distribution actually look like? Numbers summarize; pictures reveal shape, outliers and comparisons at a glance.
Open the sample (Ctrl+Shift+O). Charts land as cards in the Output View,
interleaved with tables; right-click a card to copy it as an image straight
into a document.
Histogram — one scale variable's shape
Graphs → Histogram… (Ctrl+Shift+H), variable BMI, Run.
A roughly bell-shaped pile centred in the mid-20s with a right tail — real BMI data look like this too. Re-run with the bins slider at different values: more bins show more detail and more noise. The bins are display only — they change no data. (Turning BMI into actual categories is a transform — module 8.)
Boxplot — a scale variable across groups
Graphs → Boxplot… (Ctrl+Shift+B), variable SystolicBP, category
SmokingStatus, Run.
Each box is a group's median and quartiles, whiskers and outlier points. The Current box should sit visibly higher — the picture version of the test you will run in module 6.
Line Chart — a trend across a scale variable
Graphs → Line Chart… (Ctrl+Shift+L), Y series SystolicBP, X axis
Age, split by SmokingStatus, Run.
health_study is one row per participant, not one row per visit, so this connects each smoking group's points in age order rather than tracing a single patient over time — the mechanics are the same chart you would use for a real time series (repeated visits, a stock price, a sensor log). Turn on point markers to see the individual participants along each line.
The Chart Builder
Graphs → Chart Builder… (Ctrl+Shift+G) is the general dialog: pick the
chart type, assign variables to its roles. Rebuild the boxplot above in it,
then try a Scatterplot of BMI (X) against SystolicBP (Y) — the cloud
should visibly climb, previewing module 7's correlation.
Exercise
- Histogram of FastingGlucose_mmolL. There is a small second bump well to
the right of the main pile — what is it? (Codebook: a diabetic-range
cluster.) Note that the declared-missing
99s are not in the plot. - Boxplot of BMI by Sex. Do the medians differ much?
- Scatterplot Age (X) vs HeartRate (Y). Describe what "no relationship" looks like — you will want this picture in mind whenever a correlation comes out near zero.
The syntax trail
histogram variable=BMI
boxplot variable=SystolicBP, category=SmokingStatus
line_chart y=[SystolicBP], x=Age, split=SmokingStatus
scatter x=BMI, y=SystolicBP