function result = onlineVariance(data)
  n = 0;
  mean = 0;
  M2 = 0;

  for i = 1 : size(data,2)
    x  = data(i);
    n  = n + 1;
    delta = x - mean;
    mean = mean + delta / n;
    M2 = M2 + delta*(x-mean);
  end

  if n < 2;
    result = -1;
  else
    result = M2 / (n-1);
  end