src/collectors/powerSumAvg_running.js

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