From The Free Forex Encyclopedia
The disparity index is a percentage measurement for the position of the current closing price of an asset relative to that asset's moving average. Traders commonly attribute this measurement to Steve Nison, based on his book Beyond Candlesticks.
The disparity index can take either a positive or a negative value. A positive value indicates that the asset's price is rapidly increasing, while a negative value indicates that the price is rapidly decreasing. A value of zero means that the asset's current price is exactly consistent with its moving average.
The disparity index crossing the zero line reflects an extremely rapid change in the trend of a given asset, and is therefore a strong early-warning indicator of the asset's increasing momentum.
Nison's book suggests that the disparity index can indicate whether an asset is overbought (in the case of a positive value) or oversold (in the case of a negative.) Since overbought and oversold assets are very vulnerable to rapid price reversals, the disparity index is a good indicator of when following the trend of a given asset might be a dangerous proposition.