src/collectors/variance_running.js

 /**
  * Returns the definition of a collector that captures `variance_running`
  *   - Modified from the following source:
  *     https://subluminal.wordpress.com/2008/07/31/running-standard-deviations/
  * @return {Object} A `variance_running` collector object
  */
export default function () {
  return {
    name: 'variance_running',
    initialValue: 0,
    requirements: ['count', 'mean', 'powerSumAvg_running'],
    onGet: function (prev, state) {
      return (state.powerSumAvg_running * state.count -
        state.count * state.mean * state.mean) / (
        state.count - 1);
    }
  };
}