code: function trackingSignal(actuals, estimates) {
                const runningSumOfForecastErrors = sum(forecastErrors(actuals, estimates));
                const MAD = meanAbsoluteDeviation(actuals, estimates);
                return runningSumOfForecastErrors / MAD;
              }
docs: Tracking Signal - Used to pinpoint forecasting models that need adjustment
  @memberOf util
  @see {@link https://scm.ncsu.edu/scm-articles/article/measuring-forecast-accuracy-approaches-to-forecasting-a-tutorial}
  @example
  const actuals = [ 45, 38, 43, 39 ];
  const estimates = [ 41, 43, 41, 42 ];
  const trackingSignal = ms.util.trackingSignal(actuals, estimates);
  trackingSignal.toFixed(2) // =>  -0.57
  @param {Number[]} actuals - numerical samples
  @param {Number[]} estimates - estimates values
  @returns {Number} trackingSignal