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Proportional stratified allocation gives each stratum a share of the sample matching its share of the population: nₕ = n·Nₕ/N. This guarantees representativeness and usually reduces variance relative to simple random sampling.
Proportional allocation
nₕ = n · Nₕ / N
The allocation is 80, 120 and 200 — proportional to the 20%, 30% and 50% shares.
When you need comparable precision within each stratum, such as comparing small subgroups against large ones.