Chart questions

Numerical reasoning practice > Chart questions

Charts look friendlier than tables and cost more marks. The reason is that you have to read a value off an axis instead of being given it, and small misreadings compound.

1. Read the axis before the bars

The first thing to look at on any chart is not the data. It is the scale.

• Where does the axis start? An axis starting at 200 rather than 0 makes a small difference look enormous
• What is each gridline worth? Fifty? Two hundred?
• What is the unit? Thousands and millions are written small

A question that asks "how many times larger" is often testing whether you noticed a truncated axis.

2. What each chart type is really testing

• Bar charts test comparison. Differences, highest and lowest, ranking
• Line graphs test change over time. Direction, rate of change, when a trend turned
• Pie charts test proportion. Share of a whole, and converting a percentage back into an absolute number

Knowing which of the three you are looking at tells you what the answer will be shaped like before you read the question.

3. Worked example: a range from a bar chart

The chart below shows the annual revenue of each division.

What is the difference between the highest and the lowest revenue?

How to work it

• "Difference between highest and lowest" is the range. You need exactly two bars
• Find the tallest and the shortest by eye. Do not read every bar
• Read both values off the axis, then subtract
• If the answer came to 340 and the unit note said £000s, the answer is 340 thousand, not 340

The work here is two readings and one subtraction. Anything more means you have misread the question.

4. Pie charts: the percentage is not the answer

Pie chart questions usually give you a total somewhere in the text and a set of percentages in the chart. The answer is almost never the percentage itself.

• "What share does X have?" — the percentage is the answer
• "How much revenue did X generate?" — multiply the percentage by the total
• "How much more than Y?" — take the difference in percentage points first, then multiply once. Multiplying each and subtracting works but takes twice as long

When two pie charts are shown side by side, check whether the totals are the same. If they are not, a larger slice can represent a smaller number.

5. When to estimate and when not to

Reading a value off a chart is always approximate, so the test cannot expect exactness. Look at the options:

• Options far apart (200 / 340 / 500) — estimate confidently
• Options close together (338 / 340 / 342) — the question is not really about the chart; there is a stated figure somewhere you are meant to use