Here is an example in C++ code of one way you might implement a calculation of the Sample Variance. It may not be completely optimized, but it should be fairly efficient:
double sample_variance(double *signal, unsigned int signal_length) { double sum = 0.0; double sum2 = 0.0; for (unsigned int i=0; i<signal_length; i++) { sum += signal[i]; sum2 += signal[i]*signal[i]; } double N = (double)signal_length; return (sum2 - sum*sum/N)/(N-1.0); }