public double std() {
if (s0 > 0) {
Vector radical = s2.times(s0).minus(s1.times(s1));
radical = radical.times(radical).assign(new SquareRootFunction());
Vector stds = radical.assign(new SquareRootFunction()).divide(s0);
return stds.zSum() / stds.size();
} else {
return 0;
}
}