Statistical diagrams

WJEC tests one diagram per Unit 2 paper at Foundation, sometimes two. The skill is accurate construction (with a key/labels) and interpretation (reading off and computing).

Bar charts

• Equal width bars; gaps between bars (categorical data).
• Both axes labelled; title.
• Frequency on y-axis. Discrete data on x-axis.

Dual / composite bar charts

Side-by-side bars compare two groups. A key is essential.

Pictograms

Use a symbol to represent a fixed frequency. Always include a key (e.g. "★ = 4 cars"). Half symbols allowed for half the unit.

WJEC Foundation common error: forgetting the key. 1 mark lost.

Pie charts

Each slice's angle is proportional to the category's frequency.

Angle = (frequency ÷ total) × 360°.

Example: 30 students; 12 chose football. Angle = (12/30) × 360° = 144°.

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

WJEC pie-chart construction marks:

• M1 — at least one angle correct.
• A1 — all angles correct.
• A1 — chart drawn accurately (within ±2°), labelled.

Frequency tables (revisited)

Already covered in P1. Key formulas:

• Mean = Σ(fx) ÷ Σf.
• Mode = value with highest frequency.
• Range = max − min (use original data, not the table totals).

Stem-and-leaf diagrams (Foundation/Intermediate)

Each row shows a stem (tens) and leaves (units in ascending order). Always include a key (e.g. "2 | 5 means 25").

WJEC exam tip

For all diagrams: title, both axis labels, units (if applicable), and a key. Each is independently mark-bearing on Foundation.