lib/bench.js
function recordSpeed (child, speed) {
    child.runCount += sampleSize
    if (passCount > 1) {
      var square = Math.pow(speed - child.speed, 2)
      child.variance += (square - child.variance) / (passCount - 1)
    }
    child.speed += (speed - child.speed) / passCount
  }
function recordSpeed (child, speed) {
    child.runCount += sampleSize
    if (passCount > 1) {
      var square = Math.pow(speed - child.speed, 2)
      child.variance += (square - child.variance) / (passCount - 1)
    }
    child.speed += (speed - child.speed) / passCount
  }
Incorporate a speed sampling into the mean speed and (running) variance.