Doubling time is calculated by 72 divided by the rate of growth (r). Fertility rate: measures x children being born to women between 15 and 45 years of age. Productive sector: between ages of 18 and 65 in first world, and between 10 and 45 in the fourth and fifth worlds. Growth Rate: (Immigration - Emmigration) + (Birth Rate - Death Rate) Example: 5.6% Immigration 2.3% Emmigration 4.9% Birth Rate 3% Death Rate Migration rate: 5.6% - 2.3% = 3.3% Natural increase: 4.9% - 3% = 1.9% Growth rate: 3.3% + 1.9% = 5.2% 72/5.2 = 13.88 = ~14 years