A measurement system is used to measure voltage ten times. Select the most appropriate equation to be used to calculate the standard deviation value of the data.
1) σ = √(Σ(x - μ)² / N)
2) σ = √(Σ(x - μ)² / (N - 1))
3) σ = √(Σ(x - μ) / N)
4) σ = √(Σ(x - μ) / (N - 1))
