The main difference between fast and slow stochastics is summed up in one word: sensitivity. The fast stochastic is more sensitive than the slow stochastic to changes in the price of the underlying security and will likely result in many transaction signals. However, to really understand this difference, you should first understand what the stochastic momentum indicator is all about.
How the Stochastic Momentum Oscillator Works
Developed in the late 1950s, the stochastic momentum oscillator is used to compare where a security's price closed relative to its price range over a given period of time—usually 14 days. It is calculated using the following formula:
%K=(H14−L14)100∗(CP−L14)where:C=Most recent closing priceL14=Low of the 14 previous trading sessions
A %K result of 80 is interpreted to mean that the price of the security closed above 80% of all prior closing prices that have occurred over the past 14 days. The main assumption is that a security's price will trade at the top of the range in a major uptrend. A three-period moving average of the %K called %D is usually included to act as a signal line. Transaction signals are usually made when the %K crosses through the %D.
Generally, a period of 14 days is used in the above calculation, but this period is often modified by traders to make this indicator more or less sensitive to movements in the price of the underlying asset.
The result obtained from applying the formula above is known as the fast stochastic. Some traders find that this indicator is too responsive to price changes, which ultimately leads to being taken out of positions prematurely. To solve this problem, the slow stochastic was invented by applying a three-period moving average to the %K of the fast calculation. Taking a three-period moving average of the fast stochastic's %K has proved to be an effective way to increase the quality of transaction signals; it also reduces the number of false crossovers. After the first moving average is applied to the fast stochastic's %K, an additional three-period moving average is then applied—making what is known as the slow stochastic's %D. Close inspection will reveal that the %K of the slow stochastic is the same as the %D (signal line) on the fast stochastic.
The Bottom Line
An easy way to remember the difference between the two technical indicators is to think of the fast stochastic as a sports car and the slow stochastic as a limousine. Like a sports car, the fast stochastic is agile and changes direction very quickly in response to sudden changes. The slow stochastic takes a little more time to change direction but promises a very smooth ride.
Mathematically, the two oscillators are nearly the same except that the slow stochastic's %K is created by taking a three-period average of the fast stochastic's %K. Taking a three-period moving average of each %K will result in the line that is used for a signal.