SMI was created by William Blau in January 1993 issue of Technical Analysis of Stocks & Commodities. The SMI demonstrates where the close is relative to the middle of the last high/low range, in comparison to the close relative to the recent low/high with the Stochastic Oscillator, which resembles the Stochastic Momentum Index.
It's an oscillator that shifts between -100 and +100 and can be a bit less inconstant than an equal period Stochastic Oscillator. The oscillator consists of 2 lines - the moving average of the SMI (red) and the SMI (blue). The SMI will be negative if the close is less than the middle point of the range. The SMI will be positive if the close is greater than the middle point of the range.
The SMI interpretation is in fact the same as that of the Stochastic Oscillator. The most ordinary way of using it is to trade from is to sell when the SMI rises above +40 and then returns to the point under that level and to purchase at the moment when the SMI decreases under -40 and then shifts back above it. Another trading sign is to purchase when the SMI shifts above the moving average, and sell when the SMI decreases below the moving average.
Usually before basing any trades on strict oversold or overbought levels it is better to qualify the trendiness of the market using an indicator, for example, R-Squared. Levels should provide the most effective results if indicators provide a non-trending market trades based on strict oversold or overbought.