Williams %R is a momentum indicator developed by Larry Williams that measures overbought/oversold levels.
The formula used to calculate Williams' %R is similar to the Stochastic Oscillator :
Williams %R = - 100 * ((Highest High (n) - Close) / (Highest High (n) - Lowest Low (n)))
The standard Williams %R works with a 14-days parameter : n = 14
This Indicator was validated by the data available from comdirect.de: The DAX at 04.06.2003 (data from yahoo.com) had a Williams %R of -5.99. This is consistent with this indicator: -5.99328026152501
http://www.equis.com/free/taaz/williamspercr.html