The basic Directional Movement trading system involves comparing the 14-day +DI
("Directional Indicator") and the 14-day -DI. This can be done by plotting the two
indicators on top of each other or by subtracting the +DI from the -DI. Wilder suggests
buying when the +DI rises above the -DI and selling when the +DI falls below the -DI.
Wilder qualifies these simple trading rules with the "extreme point rule." This rule is
designed to prevent whipsaws and reduce the number of trades. The extreme point rule
requires that on the day that the +DI and -DI cross, you note the "extreme point." When
the +DI rises above the -DI, the extreme price is the high price on the day the lines
cross. When the +DI falls below the -DI, the extreme price is the low price on the day the
lines cross.
The extreme point is then used as a trigger point at which you should implement the
trade. For example, after receiving a buy signal (the +DI rose above the -DI), you should
then wait until the security's price rises above the extreme point (the high price on the
day that the +DI and -DI lines crossed) before buying. If the price fails to rise above
the extreme point, you should continue to hold your short position.
In Wilder's book, he notes that this system works best on securities that have a high
Commodity Selection Index (page Error! Bookmark not defined.). He says, "as a rule of
thumb, the system will be profitable on commodities that have a CSI value above 25. When
the CSI drops below 20, then do not use a trend-following system."
