# stdev

## stdev(T.a : number) 🡒 number, aggregator

Returns the stand deviation on a population. The formula is given by:

$$s = \sqrt{\frac{1}{N - 1} \sum_{i=1}^N \left(x_i - \bar{x}\right)^2 }$$

where $\textstyle{x_1,, x_2,, \ldots,, x_N}$ are the observed values of the sample items, $\bar{x}$ is the mean value of these observations, and $N$ is the number of observations in the sample.

Example:

table T = with
[| as A, as B |]
[| 0,   "a"   |]
[| 0,   "a"   |]
[| 0,   "b"   |]
[| 1,   "b"   |]
[| 2,   "c"   |]

table G[gdim] = by T.B

where T.B != "c"
show table "" a1b4 with
gdim
stdev(T.A)
group by gdim


The aggregator returns 1 on empty groups.