To understand how these two indicators can be used together, we must first for a moment review each of them. Momentum is the measurement of the speed or velocity of price changes. In “Technical Analysis of the Financial Markets”, John J. Murphy explains:
“Market momentum is measured by continually taking price differences for a fixed time interval. To construct a 10-day momentum line, simply subtract the closing price 10 days ago from the last closing price. This positive or negative value is then plotted around a zero line. The formula for momentum is:
M = V – Vx
V is the latest price, and Vx is the closing price x number of days ago.”
Momentum measures the rate of the rise or fall in stock prices. From the standpoint of trending, momentum is a very useful indicator of strength or weakness in the issue’s price. History has shown us that momentum is far more useful during rising markets than during falling markets; the fact that markets rise more often than they fall is the reason for this. In other words, bull markets tend to last longer than bear markets.
For relative strength, determining the true value of an oscillator depends on the understanding of overbought or oversold positions. (See Speed Resistance Lines.) There has always been a little confusion over the difference between relative strength, which measures two separate and different entities by means of a ratio line, and the relative strength index, which indicates to the trader whether or not an issue’s price action is created by those over-buying or over-selling it. The well-known formula for the relative strength index is as follows:
RS = Average of x days’ up closes Average of x days’ down closes
At the bottom of the chart, the RSI, on a scale of 0-100, indicates that the overbought position is at 70 and the oversold position is at 30. An trader with today’s simple-to-use software may choose to reset the indicators’ parameters to 80 and 20. This helps the trader be sure when making the decision to buy or sell an issue and not pull the trigger too fast.
I have always found that the RSI works best when compared to short-term moving-average crossovers. Using a 10-day moving average with a 25-day moving average, you may find that the crossovers indicating a shift in direction will occur very closely to the times when the RSI is either in the 30/70 or 20/80 range, the times when it is showing either distinct overbought or oversold readings. Simply put, the RSI forecasts sooner than almost anything else an upcoming reversal of a trend, either up or down.