This has become one of the most widely used and popular of technical indicators. It was invented by Welles-Wilder, and uses a simple equation comparing the average up moves in the market to the average downmoves to give a single RSI number for a certain period. The 14-day RSI is widely used. RSIs of 20-30% tend to indicate the market is oversold, while those of 70-80% indicate it is overbought.