The famous Dow theory for timing purchases and sales

1. The famous Dow theory for timing purchases and sales has had an unusual history.
2. Briefly, this technique takes its signal to buy from a special kind of "breakthrough" of the stock averages on the upside, and its selling signal from a similar breakthrough on the downside.
3. The calculated - not necessarily actual - results of using this method showed an almost unbroken series of profits in operations from 1897 to the early 1960s.
4. On the basis of this presentation the practical value of the Dow theory would have appeared firmly established; the doubt, if any, would apply to the dependability of this published "record" as a picture of what a Dow theorist would actually have done in the market.
5. A closer study of the figures indicates that the quality of the result shown by the Dow theory changed radically after 1938 - a few years after the theory had begun to be taken seriously on Wall Street.
6. Its spectacular achievement had been in giving a sell signal, at 306, about a month before the 1929 crash and in keeping its followers out of the long bear market until things had pretty well righted themselves, at 84, in 1933.
7. But from 1938 on the Dow theory operated mainly by taking its practitioners out at a pretty good price but then putting them back in again at a higher price. For nearly 30 years thereafter, one would have done appreciably better by just buying and holding the DJIA.
8. In our view, based on much study of this problem, the change in the Dow-theory results is not accidental. It demonstrates an inherent characteristic of forecasting and trading formulas in the fields of business and finance.
9. Those formulas that gain adherents and importance do so because they have worked well over a period, or sometimes merely because they have been plausibly adapted to the statistical record of the past.
10. But as their acceptance increase, their reliability tends to diminsh.
11. This happens for two reasons: First, the passage of time brings new conditions which the old formula no longer fits.
12. Second, in stock-market affairs the popularity of a trading theory has itself an influence on the market's behaviour which detracts in the long run from its profit-making possibilities.
13. (The popularity of something like the Dow theory may seem to create its own vindication, since it would make the market advance or decline by the very action of its followers when a buying or selling signal is given. A "stampede" of this kind is, of course, much more of a danger than an advantage to the public trader.)

Ref: Intelligent Investor by Benjamin Graham