The primary trend wave indicates the fundamental direction of market price movements. It can also be referred to as a "bull market" or "bear market." Its duration may be less than one year or extend for several years. Correctly identifying the direction of the primary trend wave is the most critical factor in successful investment (or speculation). To date, there is no method that can accurately predict the duration or price amplitude of the primary trend wave.

Dow's Theorem V contains two noteworthy points:

First, the extreme importance of correctly identifying the primary trend wave for successful investment;

Second, there is no method that can accurately predict the duration or price amplitude of the primary trend wave. A hundred years ago, Dow stated that no method could accurately predict the length of the primary trend wave, and a hundred years later, there remains no method to do so. This is a profoundly important philosophical concept. If investors accept that no method can accurately predict the length of the primary trend wave, their investment approach should be based on trend-following techniques rather than trend-prediction techniques.