威廉指数（Williams%Rate）是由拉瑞·威廉于1973年的《我如何赚取百万美元》一书中首先发表的，所以以他的名字命名，原名叫“威廉超买超卖指数”，简称WMS%R或%R。该指数是利用摆动点来度量市场的超买超卖现象，所以可以此预测循环周期内的高和低点，找出其有效信号，是分析市场短期行情走势的技术指标。计算时先要决定周期天数，用此数取一个买卖循环的半数。许多分析家认为一个买卖循环为28天，除开周六、日，实际交易日为20天。一个较长的买卖循环期为56天，除开周六、日，交易日是40天，如各取其一半正好是10天和20天。也有取5天周期来计算%R的

In technical analysis, this is a momentum indicator measuring overbought and oversold levels, similar to a stochastic oscillator. It was developed by Larry Williams and compares a stock's close to the high-low range over a certain period of time, usually 14 days.

It is used to determine market entry and exit points. The Williams %R produces values from 0 to -100, a reading over 80 usually indicates a stock is oversold, while readings below 20 suggests a stock is overbought.