Williams %R (pronounced “percent R”) is a momentum indicator developed by Larry Williams. It oscillates between 0 and -100. Readings in the -80 to -100 range indicate an oversold condition while readings in the 0 to -20 range indicate an overbought condition.
Like many “overbought/oversold” oscillators it is best to wait for the underlying's price to change direction and for the indicator to reverse course to confirm the move.
The percent R has been known to be very good at reversing a few days before prices do (i.e. it peaks a few days before a top is achieved and bottoms a few days before the underlying reverses). But it is not uncommon for an overbought or oversold indicator to stay overbought or oversold for a long period of time, so one needs to study the history of the percent R with a stock to determine its tendencies.