I like Barry Ritholtz’ The Big Picture, but today he makes an odd claim:
That is a new slant on timing; we’ve evolved from “You cannot time the market” to “most find managers cannot time the market” — a subtle but important distinction.
New? Wasn’t it Charles Dow who dealt out playing cards with stock names on them, and matched the performance of investment advisors, back in the 1920’s? Apparently playing cards predate the dart-board metaphor, but under either name it’s ancient knowledge.
This “new” (irony quotes) fight between buy-and-holders and traders makes me step back, and to think about what we can know about our performance past, present and future. I generally hang with buy-and-holders, and index investors, but I’ve noticed a tension even within that group. Generally back-testing, the testing of a portfolio against decades of market history, is considered good. At the same time, data mining, the process of drilling down in recent market history to find optimum investments and strategies is considered bad. There is some merit in the distinction. At some point mining can become over-mining. I think everyone would agree with that. We can trust history too much, and trap ourselves into thinking that a relationship that was true in the past is likely to be true in the future.
We back-test (or use more arcane technical analysis) even as we say “past performance does not guarantee future results.”
This tension between looking at history and not trusting history is the key to understanding not just the tension between buy-and-holders and traders, but the uncertainty within each group as well.
I think everything is data mining, and that everyone should be using the past as their model with great reluctance.
If you think otherwise … you might become at the minimum a slice-and-dice buy-and-holder, or at the maximum, a trader.
UPDATE: One thing you can say about back-testing is that it lets you say a strategy “at least” works in the past. It may not work in the future, but it probably is safer than a strategy that “did not even” work in the past.