The mean square deviation of a set of observations \(x_1,x_2,\ldots,x_n\) about point \(c\) is defined as
\[
\frac1n\sum_{i=1}^n(x_i-c)^2.
\]
The mean square deviations about \(-2\) and \(2\) are 18 and 10 respectively. The standard deviation of the set of observations is