Reading graphs and scales: the half-mark mistake
The maths is easy; the reading is where the mark goes. A bar ending halfway between 20 and 30 is read as 21 because each gap "looks like 1", the wrong row of a table is used, or a line graph is read at the wrong point on the axis.
What it looks like
A bar graph shows the books read by four pupils in a month. The axis is marked 0, 10, 20, 30 and 40. Ali’s bar ends exactly halfway between 20 and 30. How many books did Ali read?
- Each mark is 10 books, so halfway between two marks is 5 books.
- Halfway between 20 and 30 is 25.
Why 21 is wrong: The gap between marks was read as 1 instead of 10, so a bar halfway up the gap became 21.
Why children do it
- Scales in the PSLE rarely go up in ones. A gap can be worth 2, 5, 10 or 50, and the value has to be worked out, not assumed.
- Tables with several columns invite reading the right row in the wrong column.
- Questions often ask for a change (from Monday to Wednesday) rather than a value, and only one of the two is read.
How to fix it at home
- Before reading any value, have your child write what one gap on the axis is worth.
- Use a ruler or the edge of a paper to carry the top of a bar across to the axis.
- For tables, underline the row and the column heading before taking the number.
Questions parents ask
Which graphs appear in the PSLE?
Tables, bar graphs, line graphs and pie charts. Pie charts are the P6 item; the others have been built up since P3.
Is this really a maths habit?
It costs maths marks, so yes. It is also the one most easily fixed at home, with any chart in a newspaper.
How does Knowbotic catch it?
Misread values are known traps on graph questions, so a wrong answer is named as this habit and practised until fixed.
Where it shows up
Other careless mistakes
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