Population Dynamics
Five rates — CBR, CDR, RNI, TFR, and IMR — are the vocabulary geographers use to describe whether, and how fast, a population is growing. Confusing them is one of the most common exam mistakes.
Crude Birth Rate (CBR) and Crude Death Rate (CDR) count live births and deaths per 1,000 people in a population per year. Subtract one from the other and you get the Rate of Natural Increase (RNI) — a country with a CBR of 40 and a CDR of 10 has an RNI reflecting a gap of 30 per 1,000, a much faster-growing population than one with a CBR of 20 and a CDR of 15.
Total Fertility Rate (TFR) is a different kind of number — the average number of children a woman is expected to have over her lifetime, rather than a population-wide rate. A TFR around 2.1 is considered replacement level: roughly enough to replace both parents once infant and childhood deaths are accounted for. Below that, a population eventually starts shrinking.
Infant Mortality Rate (IMR) counts deaths of children under age 1 per 1,000 live births, and it’s one of the clearest signals of a country’s healthcare quality and living conditions. IMR and TFR are also connected: where infant mortality is high, families often have more children as a buffer against expected loss — so falling IMR tends to go hand in hand with falling TFR as a country develops.