Statistical charts and graphs

Bar charts

Used for categorical or discrete data. Bars are the same width with gaps between them. The y-axis shows frequency. Compound (multiple) bar charts compare two groups side by side.

Pie charts

Show proportion. The angle for each category = (frequency ÷ total) × 360°.

Example: 60 students, 15 study French. Angle = 15/60 × 360° = 90°.

To read a pie chart, reverse the formula: frequency = (angle ÷ 360°) × total.

Frequency diagrams (continuous data, equal class widths)

Like a bar chart but with no gaps — used for grouped continuous data such as heights or times.

Histograms with unequal class widths (Higher only — see S4)

Frequency density = frequency ÷ class width. Plot frequency density on the y-axis. Area of each bar represents frequency.

Scatter graphs and correlation

Plot pairs (x, y). Correlation describes how the two variables move together.

• Positive correlation: as x increases, y increases (line of best fit slopes up).
• Negative correlation: as x increases, y decreases.
• No correlation: random scatter.

A line of best fit passes through the centre of the data with roughly equal points either side. Use to estimate y given x within the data range (interpolation). Don't extrapolate.

Box plots (Higher tier — see S6)

Show the five-number summary: min, Q1, median, Q3, max. The box is from Q1 to Q3; the line inside is the median. Whiskers extend to min and max. Compare two distributions by comparing centre (median) and spread (IQR).

Common CCEA exam tip

Always title your chart, label axes (with units), and use a sensible scale. The B1 communication mark depends on it.