docs: function groupStats(indexes) {
    var N = indexes.length
    var avg = indexes.reduce(function (a, b) {return a + b}, 0) / N
    return {
      group: indexes,
      avg: avg,
      early: Math.sqrt(avg),
      stddev: Math.sqrt(indexes.reduce(function (v, e) {
          return v + Math.pow(e - avg, 2)
        }, 0) / N),
     }
  }