A population changes in only four ways: births and immigrants add people; deaths and emigrants subtract them. Demographers start with births and deaths. Because countries differ hugely in size, they compare rates, not raw counts, using the idea of a percent and its cousin, "per 1,000."
Key idea
The crude birth rate (CBR) is births per 1,000 people in a year. The crude death rate (CDR) is deaths per 1,000 people in a year. The rate of natural increase (RNI) is CBR − CDR, usually written as a percent: divide the difference by 10.
Why it works
"Per 1,000" is a fraction with 1,000 on the bottom; a percent has 100 on the bottom. So 20 per 1,000 = 2 per 100 = 2%. That is why you divide by 10. The word natural means the rate leaves out migration: it counts only births and deaths. And crude means the rates use the whole population, of every age, as the base.
Worked examples
Worked example: From counts to rates
A country of 4,000,000 people had 96,000 births and 28,000 deaths last year. Find CBR, CDR and RNI.
- CBR = 96,000 ÷ 4,000,000 × 1,000 = 24 per 1,000.
- CDR = 28,000 ÷ 4,000,000 × 1,000 = 7 per 1,000.
- Difference: 24 − 7 = 17 per 1,000.
- As a percent: 17 ÷ 10 = 1.7%.
Answer: CBR 24, CDR 7, RNI 1.7% a year.
Worked example: A shrinking population
CBR 8, CDR 11. What is the RNI?
- 8 − 11 = −3 per 1,000.
- −3 ÷ 10 = −0.3%.
- Negative natural increase: more deaths than births.
Answer: −0.3% a year (natural decrease).
Common mistake
Students subtract and stop, writing "RNI = 20%" for CBR 30 and CDR 10. The difference is 20 per 1,000, which is 2.0 percent. Check the size: a 20% growth rate would double a population in under four years, which no country does. Real natural increase rates are between about −1% and 4%.
| CBR | CDR | Per 1,000 | RNI |
|---|---|---|---|
| 40 | 20 | 20 | 2.0% |
| 25 | 7 | 18 | 1.8% |
| 12 | 9 | 3 | 0.3% |
| 9 | 12 | −3 | −0.3% |