A chart shows the shape of data quickly. Choose the chart by the question and the variable type.
Why it matters
A good plot often reveals outliers, skew and group differences that a table of means hides.
Key ideas
- Bar chart: categories
- Histogram: one numeric variable's shape
- Box plot: median, quartiles, outliers, group comparison
- Scatter plot: two numeric variables
- Line chart: change over time
- Start bar axes at zero
Real-life research example
To compare systolic BP of two clinics, side-by-side box plots show median, spread and outliers at once.
When to use
- Plot before testing, and again when reporting
When NOT to use
- Don't use 3-D or pie charts for many slices
- Don't cut the y-axis to exaggerate a difference
Mini example
Values: 4, 5, 5, 6, 7, 8, 40 → the dot plot shows one point far to the right
Quick takeaway
Plot first — your eyes catch what numbers hide.
Try it yourself
Type numbers with one extreme value. Look at the dot plot: where do mean and median sit, and which point is isolated?
Model answer
The mean (dashed) is pulled toward the extreme point; the median (solid) stays with the main group. The isolated point is the outlier.