Bar charts and pictograms
- Bar charts show qualitative or discrete data. Bars have equal widths and gaps between them.
- Composite (stacked) bar charts show how a total splits into parts. Multiple (comparative) bar charts put bars side by side to compare groups.
- Pictograms use symbols, with a key such as "one symbol = 4 people".
Pie charts
- A pie chart shows how a whole divides into parts. Angle = frequency ÷ total × 360°. If 15 of 60 people chose blue, the angle is 90°.
- Pie charts show proportions, not actual numbers.
- Comparative pie charts (higher): the areas are in proportion to the totals, so radius ratio = √(total ratio). If one chart shows 100 people with radius 4 cm, a chart for 225 people has radius 4 × √(225 ÷ 100) = 6 cm.
Stem-and-leaf and other diagrams
- A stem-and-leaf diagram keeps the raw data and puts it in order. It must have a key, such as 3 | 5 means 35. Back-to-back stem-and-leaf diagrams compare two sets.
- Population pyramids show the age and gender structure of a population.
- Choropleth maps shade areas according to a value, with darker shades for higher values.
- Line graphs show changes over time.
Misleading diagrams
- Watch for axes that don't start at zero, uneven scales, missing labels, 3D effects that distort size, and pictures scaled in two directions.
Key terms
- Bar chart
- A chart using bars of equal width with gaps.
- Composite bar chart
- A bar chart showing how totals divide into parts.
- Pie chart
- A circle divided into sectors to show proportions.
- Stem-and-leaf diagram
- A diagram that orders raw data using stems and leaves.
- Population pyramid
- A chart showing a population's age and gender structure.
- Choropleth map
- A map shaded to show values in each area.
- Key
- An explanation of what symbols or values in a diagram mean.