Presenting data: bar graphs and pie charts
A bar graph compares amounts with bars of equal width. A pie chart shows parts of a whole: each sector's angle is value ÷ total × 360°.
Types of bar graph
| Type | What it shows | Nutrition example |
|---|---|---|
| Simple bar | One variable across categories; one bar per category | Anaemia prevalence in four schools |
| Multiple (grouped) bar | Two or more bars side by side for each category | Boys' and girls' anaemia in each class |
| Sub-divided (component) bar | One bar split into parts that add up to the total | Girls in each school split into normal, mild, moderate and severe anaemia |
| Percentage bar | A sub-divided bar where every bar is 100% | Share of energy from cereals, pulses and fats in three diets |
| Deviation bar | Positive and negative values above and below a base line | Weight gained or lost after a diet programme |
| Broken bar | One very large value; its bar is broken to save space | One district with far more cases than the rest |
Rules for drawing a bar graph
- Keep all bars the same width, with equal gaps between them.
- Start the value axis at zero, so bar length is true to the value.
- Choose a scale that fits the largest value on the page.
- Bars can be vertical or horizontal; arrange them in a logical order (size, time or place).
- Give a title, label both axes with units, and add a key for multiple or sub-divided bars.
- Names vary: some books call the sub-divided bar a 'component' bar and the multiple bar a 'grouped' or 'compound' bar.
Pie chart angles: anaemia grades
Of 120 adolescent girls tested in a school, 45 are normal, 39 have mild anaemia, 33 moderate and 3 severe (WHO Hb cut-offs). Find the angle of each sector for a pie chart.
- Total = 45 + 39 + 33 + 3 = 120 girls
- Each girl = 360° ÷ 120 = 3°
- Normal: 45 ÷ 120 × 360° = 135°
- Mild: 39 ÷ 120 × 360° = 117°
- Moderate: 33 ÷ 120 × 360° = 99°
- Severe: 3 ÷ 120 × 360° = 9°
- Check: 135 + 117 + 99 + 9 = 360°
- Draw: start at 12 o'clock, go clockwise, largest sector first; label each with its percentage (37.5%, 32.5%, 27.5%, 2.5%) and add a key.
Sectors of 135°, 117°, 99° and 9°. The chart shows that 62.5% of the girls (75 of 120) are anaemic.
Bar graph vs pie chart
Do not confuse a bar graph with a histogram: bar graph bars are separate and show categories; histogram bars touch and show class intervals of continuous data.
Use a pie chart for the share of energy from cereals, pulses and fats in a diet, or for anaemia grades in a group. Use a multiple bar graph to compare anaemia before and after a mid-day meal or iron programme across schools.
2 marks: “How is the angle of a sector in a pie chart calculated?” Angle = (value ÷ total) × 360°. For example, 45 out of 120 gives 45 ÷ 120 × 360° = 135°.
5 marks: “Explain the different types of bar graphs with examples.” Simple, multiple, sub-divided, percentage, deviation and broken bars, each with one line and a small sketch or nutrition example.
10 marks: “Explain the presentation of data by bar graphs and pie charts with suitable examples.” Rules and types of bar graph with sketches; the pie-chart formula with a worked example; when to use each; a nutrition example.
A pie is one whole = 360°, so every 1% = 3.6°.