How it is calculated
The formula is for a very large population, when the sample barely depletes it. This screen does not apply a correction for a finite population.
n = z² × p × (1 − p) / e²
Where the numbers come from
- Level
- Expected proportion — unit in the field: %
- Margin of error — unit in the field: p.p.
Example
At 95%, a share of 50%, and an error of 5 percentage points, you need 385 answers.
Questions
Is an error of 5 equal to 5% or 0,05?
Five percentage points. In the formula that is 0,05.
Why exactly 385?
At z ≈ 1,96, p = 0,5, and e = 0,05 the expression gives about 384,16; rounded up, 385.
Is this the number of forms, or the number of people in a city?
The number of answers in the sample, not the size of the population.