简单搜了一下。这个方法有很多缺陷。比如没有考虑利率和结构性变化的影响。看看就好。

"Since 1990, however, the metric has only spent 2% of the time below its historical average–98% of the time above. I'd argue that any metric designed to provide signaling to the future direct of said market cannot spend 98% of its time on one side of its average over a 25-year period.“