RSI (Relative Strength Index) - Relative Strength Index (RSI) is an oscillator that measures a particular stock's current relative strength compared to its own price history. The RSI is plotted on a vertical scale numbered from 0 to 100. A security is considered to be oversold when it falls below 30 and overbought when it rises over 70. Failure swings occur when in a downtrend the RSI fails to set a new low and then goes on to set a new high or in an uptrend when the RSI fails to set a new high and then goes on to set a new low.