Simply put, this analysis holds that the Dow Jones Industrials and Transports should move in lock step to confirm each intermediate-term move in the stock market. When one index see a relative new high, so should the other. The opposite is true at lows. When one index diverges from the other, it is a possible sign of a trend change. When both indices closes below a previous significant low that is considered confirmed change of trend.
Let’s look at the two charts below for some real life, recent examples. At the far lower left of both charts you can see the Ebola October lows where both indices closed below previous significant lows. By early December, they both went right back to new highs where the party began to fizzle.
Since early December, the transports have seen a progression of peaks that were each lower than the previous one where the industrials saw a clear trend of higher highs through February. With one index lagging the other, this is called a non-confirmation or divergence and warns that all is not perfect healthy. Of course, if and when the lagging index rallies to confirm the leader, this Dow Theory warning completely goes away. Right now, we have a series of caution flags since late 2014 without much in the way of price weakness.