Actually , if we can look on activities of stock market as a series price competition and dealing process in a fixed system , then we can choose p / e as the process variable to study the venture of stock market with spc ( statistic process control ) theory 股票市場的運動演進可視為一個系統內的投資者連續競價交易的過程,因此可選擇表征股票市場的風險的市盈率作為過程變量,應用過程控制理論研究股票市場的風險。