To compare two age structures, use the same age categories and compare percentages of each town’s own total. Raw counts mostly show which town is bigger.
This lesson belongs to population growth, structure and migration. It builds on interpreting a population pyramid.
What makes a comparison fair?
A comparison is fair when both places use the same categories, the same year and the same unit. Only then does a difference in a bar mean a difference in structure.
Convert counts to percentages first: group ÷ total × 100.
Worked example: Town A and Town B
Both towns use the same three categories: 0 to 14, 15 to 64 and 65 and over.
| Group | Town A count | Town A % | Town B count | Town B % |
|---|---|---|---|---|
| 0 to 14 | 12 500 | 25% | 3 000 | 15% |
| 15 to 64 | 32 500 | 65% | 13 000 | 65% |
| 65 and over | 5 000 | 10% | 4 000 | 20% |
| Total | 50 000 | 100% | 20 000 | 100% |
Town A has more older people in count (5 000 against 4 000). Yet Town B has the older structure, since 20% of its people are 65 and over against 10% in A.
The dependency ratios look equal. Town A is (12 500 + 5 000) ÷ 32 500 × 100 = 53.8 per 100, and Town B is (3 000 + 4 000) ÷ 13 000 × 100 = 53.8 per 100. The ratios match, yet A’s dependants are mostly young and B’s are mostly old.
The mistake that costs marks
The common slip is to compare counts, or to compare groups that are not defined the same way.
- Wrong: “Town A has more elderly people, so it is the older town.”
- Right: “Town B has the larger share aged 65 and over, 20% against 10%, so its structure is older.”
If Town B had used 0 to 19 as its youngest group, the two youth figures could not be compared at all.
Check yourself
Town C has 20 000 residents: 4 000 aged 0 to 14, 14 000 aged 15 to 64 and 2 000 aged 65 and over. Convert to percentages and compare the youth share with Town A.
Answer
Town C: 4 000 ÷ 20 000 = 20% young, 14 000 ÷ 20 000 = 70% working age, 2 000 ÷ 20 000 = 10% old.
Town A has a youth share of 25%, so A is younger than C. Both have 10% aged 65 and over, so they do not differ at the top.
What to study next
Numbers show structure, but effects need careful wording. Continue with explaining possible effects without stereotyping a population group, then try the growth and migration practice set.
If you want a teacher to check your comparisons, see online one-to-one Geography tuition.