When an Economics calculation lands near the answer key but not on it, the cause is usually one of four checks: units, percentage base, base year, or rounding. This page walks through each with a worked example.
It supports the topics in the SPM Economics learning guide, especially interpreting inflation data.
What does this look like for you?
You might say, “I used the right formula, so why is my answer 2.8 and the key says 3?” Or, “My revenue per unit is 3 and the key says 3 000.” These are not memory problems. They are small slips that happen after the method is chosen.
What usually causes it?
Four slips account for most mismatches.
- Units. One figure in RM million and another in RM.
- Percentage base. Dividing by the new value instead of the old value.
- Percentage point versus percent. Reporting a difference in points as if it were a percentage.
- Base year. Comparing an index with the wrong year as 100.
A worked example with an index
An invented price index for Zenia reads 100 in 2022 (the base year), 108 in 2023 and 111 in 2024.
| Question | Working | Answer |
|---|---|---|
| Change in points, 2023 to 2024 | 111 − 108 | 3 points |
| Percentage change, 2023 to 2024 | (111 − 108) ÷ 108 | 2.78%, or 2.8% |
| Change since the base year, to 2024 | (111 − 100) ÷ 100 | 11% |
The same data gives three answers because each question uses a different base. A student who writes “3%” has used the point change as a percentage, and one who divides by 111 has used the new value as the base (3 ÷ 111 = 2.7%). Test other values in the percentage base and index explorer.
A unit example
A firm has revenue of RM4.5 million and sells 1 500 units. Revenue per unit is 4 500 000 ÷ 1 500 = RM3 000.
A student who divides 4.5 by 1 500 writes 0.003 and loses a factor of a million. The fix is to write the unit beside every number: RM4.5 million is RM4 500 000.
The four-check routine
Run these after you finish, before you move on.
- Unit check. Does every figure have a unit, and do they match?
- Base check. Did I divide by the starting value?
- Label check. Is my answer a point change, a percent change or a level? Does my label match?
- Size check. Is the answer plausible? A price index change of 280% from 100 to 103 would be a clear sign of a slip.
Keep a line in your mistake log for each slip, so you know which check to use first. The units and significant-figure checker helps with the first check.
What can I do this week?
Take five calculations you have already done and run the four checks on each. Note which check caught the slip. Most students find one check does most of the work, and that becomes the one to run first.
When might one-to-one tuition help?
If you repeat the same slip even after running the checks, a teacher watching you work can see where the habit starts, such as dividing without writing the base. That is the kind of detail a one-to-one lesson can surface, because the teacher sees your working rather than only the final number.
If that sounds useful, read about online one-to-one Economics tuition and the one-hour trial class (from RM50), which is a real taught lesson.