Numbers rarely sit exactly at their average - the standard deviation formula shows the typical distance they stray from it.
Population: σ = √[∑(xᵢ − μ)² / N]
xᵢ = elements in population
μ = population mean
N = population size
Sample: s = √[∑(xᵢ − x̄)² / (n − 1)]
xᵢ = elements in sample
x̄ = sample mean
n = sample size
The sample version is applied by bank risk analysts to measure day-to-day swings in currency exchange rates.