如何在计算字段中使用上个月的值

Question

我想在Tableau中通过创建一个计算字段进行人头预测，该字段考虑了未来的已知值和上个月的值。

我的表格如下：

  /* Get Our Elements */

  $('.player').each(function() {
    var player = $(this).get(0);
    var video = player.querySelector('.viewer');
    var progress = player.querySelector('.progress');
    var progressBar = player.querySelector('.progress__filled');
    var toggle = player.querySelector('.toggle');
    var skipButtons = player.querySelectorAll('[data-skip]');
    var ranges = player.querySelectorAll('.player__slider');

    /* Build out functions */
    function togglePlay() {
      const method = video.paused ? 'play' : 'pause';
      video[method]();
    }

    function updateButton() {
      const icon = this.paused ? '►' : '❚❚';
      toggle.textContent = icon;
    }

    function skip() {
      video.currentTime += parseFloat(this.dataset.skip);
    }

    function handleRangeUpdate() {
      video[this.name] = this.value;
    }

    function handleProgress() {
      const percent = (video.currentTime / video.duration) * 100;
      progressBar.style.flexBasis = `${percent}%`;
    }

    function scrub(e) {
      const scrubTime = (e.offsetX / progress.offsetWidth) * video.duration;
      video.currentTime = scrubTime;
    }

    /* Hook up the event listners */
    video.addEventListener('click', togglePlay);
    video.addEventListener('play', updateButton);
    video.addEventListener('pause', updateButton);
    video.addEventListener('timeupdate', handleProgress);

    toggle.addEventListener('click', togglePlay);
    skipButtons.forEach(button => button.addEventListener('click', skip));
    ranges.forEach(range => range.addEventListener('change', handleRangeUpdate));
    ranges.forEach(range => range.addEventListener('mousemove', handleRangeUpdate));

    let mousedown = false;
    progress.addEventListener('click', scrub);
    progress.addEventListener('mousemove', (e) => mousedown && scrub(e));
    progress.addEventListener('mousedown', () => mousedown = true);
    progress.addEventListener('mouseup', () => mousedown = false);


    $('video').on('ended', function() {
      $.fn.fullpage.moveSlideRight();
    });
  });

现在，我想创建一个字段预测，通过将已知的加入者和离开者与上个月的人数值相结合来计算未来的人数。例如：预测可能= 100 + 0 - 1 = 99

month  headcount  joiners  leavers
apr    100        4        2
may    NULL       0        1

有没有办法在Tableau中执行此操作？

提前致谢！