What it is
The 200-day moving average is the average closing price of the last 200 trading days, roughly one year of market time, redrawn each day. That's the entire indicator. It doesn't predict anything. It just tells you, honestly and a little late, which way the market has been going.
The rule built on it is just as short: when price is above the 200-day, you're allowed to be long. When price is below it, you're not. Below the line you sit in cash, or at minimum you stop opening new long positions.
Why one line works
The 200-day doesn't make you money. It stops the market from taking your money away in large, career-ending amounts. Nearly every catastrophic decline in modern market history (1929, 1973–74, 2000–02, 2008, the worst of 2020 and 2022) did the great majority of its damage below the 200-day moving average. A simple filter that steps aside when price is under the line would have missed some upside, paid some whipsaw, and skipped most of the disasters.
Mebane Faber's widely cited paper "A Quantitative Approach to Tactical Asset Allocation" tested a close cousin (the 10-month moving average, checked monthly) across stocks, bonds, commodities, and real estate going back decades. The headline result was not higher returns. It was similar returns with roughly half the maximum drawdown. For a rule you can explain to a child, that is a remarkable trade.
How to trade it without overcomplicating it
- Check it monthly, not hourly. The classic implementation looks at the monthly close only. That cuts false signals dramatically and reduces the strategy to twelve decisions a year.
- Apply it to broad indexes first. It behaves best on diversified instruments like index ETFs. Individual stocks gap, get acquired, and go to zero in ways an average can't protect you from.
- Pick a side of the line, not a feeling. Above = invested, below = cash. The moment you start adding exceptions ("it's only 1% below…"), you no longer have a system.
- Pair it with fixed risk. The filter tells you when; the 1% rule tells you how much.
What it costs you
Nothing in trading is free, and the 200-day charges its fee in three coins. First, whipsaws: in choppy, trendless markets price crosses the line repeatedly and each crossing costs a little. Second, lag: you will never sell a top or buy a bottom; the filter concedes the first chunk of every new trend by design. Third, discomfort: being in cash while the market bounces hard off a low feels terrible, and that feeling is precisely when most people abandon the rule.
Traders who accept those three costs keep the benefit. Traders who try to optimize them away (a 187-day average, a 2.3% crossing buffer, a VIX overlay) usually end up with a curve-fit system that fails live.
Common mistakes
- Watching it daily and acting on every touch. Use the monthly close, or at minimum require the signal to hold for several days.
- Treating it as a shorting signal. Below the 200-day means "stand aside," not "get short." Bear-market rallies are violent.
- Abandoning it after one bad year. Every simple system has losing years. The edge shows up across decades, not quarters.
Add one more moving average, get a full strategy
Two lines instead of one turns the trend filter into a complete entry-and-exit system.