From The Free Forex Encyclopedia
Stochastic is an indicator used in technical analysis. Stochastic compares closing prices in a market to the high and low prices for that market over a certain period of time.
Stochastic is calculated by taking the lowest low price and the highest high price for a number of previous trading periods, usually fourteen. The difference between the current closing price and the lowest low is divided by the difference between the highest high and lowest low, and the result is multiplied by 100. The product, expressed as a percentage, is considered to be the stochastic oscillator. Three varieties of this oscillator exist--fast, slow, and full--each applying a different transformation to the value of the basic stochastic oscillator.
Stochastic can be used to determine when a market is overbought or oversold. According to technical analysis, when the stochastic oscillator rises above 80%, the market is overbought, and when the oscillator drops below 20%, the market is oversold. Thus stochastic can be thought of as strong selling or buying signals, respectively.
When using stochastic, foreign exchange traders need to take into account the fact that the forex market, being a twenty-four hour market, has no closing prices. Forex traders typically use the price at the time of the New York Stock Exchange's close as the forex market's closing price, since the volume of trading drops off shortly after the close of the NYSE.