Bar charts and pictograms
- A bar chart has bars of equal width with gaps between them. The height shows the frequency. Label both axes.
- A pictogram uses a symbol for a number of items, with a key. If one symbol stands for 4 people, half a symbol stands for 2.
- Dual bar charts (side by side) and composite bar charts (stacked) compare groups.
Pie charts
- Angle = frequency ÷ total × 360°. If 12 out of 60 students choose swimming, the angle is 12 ÷ 60 × 360 = 72°.
- To read a pie chart, find the fraction of 360°. An angle of 90° is ¼ of the total.
Stem-and-leaf diagrams
- A stem-and-leaf diagram keeps the actual values, in order, with a key such as 3 | 5 = 35.
- It makes the median, mode and range easy to find.
Line graphs and frequency polygons
- A time series graph shows data over time. Look for the trend (the general direction) and repeating seasonal patterns.
- A frequency polygon plots each frequency at the midpoint of its group, and joins the points with straight lines.
- Watch out for misleading graphs, such as an axis that doesn't start at zero or an uneven scale.
Key terms
- Bar chart
- A chart using bars of equal width to show frequencies.
- Pictogram
- A chart using symbols to stand for a number of items.
- Pie chart
- A circle divided into sectors to show proportions of a total.
- Stem-and-leaf diagram
- A diagram that splits each value into a stem and a leaf, keeping the data in order.
- Key
- A note that explains what a symbol or diagram means.
- Time series
- Data recorded at regular times, shown on a line graph.
- Frequency polygon
- A graph joining points plotted at the midpoints of groups.
- Trend
- The general direction of the data over time.