The Price Rate-of-Change ("ROC") indicator displays the difference between the current price and the price x-time periods ago. The difference can be displayed in either points or as a percentage. The Momentum indicator displays the same information, but expresses it as a ratio.
It is a well recognized phenomenon that security prices surge ahead and retract in a cyclical wave-like motion. This cyclical action is the result of the changing expectations as bulls and bears struggle to control prices.
The ROC displays the wave-like motion in an oscillator format by measuring the amount that prices have changed over a given time period. As prices increase, the ROC rises; as prices fall, the ROC falls. The greater the change in prices, the greater the change in the ROC.
The time period used to calculate the ROC may range from 1-day (which results in a volatile chart showing the daily price change) to 200-days (or longer). The most popular time periods are the 12- and 25-day ROC for short to intermediate-term trading. These time periods were popularized by Gerald Appel and Fred Hitschler in their book, Stock Market Trading Systems.
The 12-day ROC is an excellent short- to intermediate-term overbought/oversold indicator. The higher the ROC, the more overbought the security; the lower the ROC, the more likely a rally. However, as with all overbought/over-sold indicators, it is prudent to wait for the market to begin to correct (i.e., turn up or down) before placing your trade. A market that appears overbought may remain overbought for some time. In fact, extremely overbought/oversold readings usually imply a continuation of the current trend.
The 12-day ROC tends to be very cyclical, oscillating back and forth in a fairly regular cycle. Often, price changes can be anticipated by studying the previous cycles of the ROC and relating the previous cycles to the current market.
The following chart shows the 12-day ROC of Walgreen expressed in percent.
The optimum overbought/oversold levels (e.g., ±6.5) vary depending on the security being analyzed and overall market conditions. I selected ±6.5 by drawing a horizontal line on the chart that isolated previous "extreme" levels of Walgreen's 12-day ROC.
When the Rate-of-Change displays the price change in points, it subtracts the price x-time periods ago from today's price:
When the Rate-of-Change displays the price change as a percentage, it divides the price change by price x-time period's ago: