In a normal distribution, the mean divides the bell-shaped curve into 2 parts: each one = 50% or 0.5
The probability within - one σ and + one σ = 68%
and from the mean to - one σ = 34%
The left par of the mean = 50% and from the mean to - one σ = 34%, then
the probability that lies below one standard deviation from the mean is:
50% - 34% = 16% or 0.16
