Building a Rules-Based Stock Portfolio and Rebalancing It

A rules-based portfolio is one where every decision follows written rules: what to own, how much, when to buy, when to sell and when to rebalance. Rebalancing means trading back to your target weights after markets have moved them, either on a calendar (for example every quarter) or when a weight drifts outside a band. Done well, it keeps your risk at the level you chose and removes most of the emotional decisions covered in trading psychology.

The seven decisions to write down

1. Allocation: how much in the core, the satellite strategy and cash.
2. Universe: which stocks the satellite may buy.
3. Selection rule: the exact signal that makes a stock eligible.
4. Slots and weighting: how many positions, and how much each gets.
5. Exits: stops, trailing stops, exit signals and time limits.
6. Rebalancing: when and how weights are brought back to target.
7. Review and kill-switch: when you stop and re-examine the whole system.

A worked blueprint

Every one of these choices is testable and every one could be different. What matters is that they are written down and that you have tested them, as in backtesting stock strategies.

Why weights drift

Different parts of the portfolio earn different returns, so their weights drift.

If you never rebalance, the best-performing part keeps growing until it dominates, and your risk becomes whatever the market made it, not what you chose.

Three ways to rebalance

Method	How it works	Pros	Cons
Calendar	Trade back to target every quarter or year	Simple, easy to follow	Trades even when drift is tiny
Bands (thresholds)	Trade only when a part is more than, say, 5 percentage points from target	Fewer trades, lower costs	Needs regular checking
Cash flow	Send new deposits and dividends to the underweight part	Few or no sales, so lower costs and often no taxable gains	Slow if deposits are small

A common hybrid: check every quarter, and trade only if something is outside its band. With a 5-point band, the 60% core above is left alone between 55% and 65%. At 64.2% it is still inside the band, so a band investor would simply direct new money to the satellite.

Rules inside the satellite

• Slot sizes: decide whether winners are trimmed back to the slot size (an equal-weight approach) or allowed to run (a trend approach). Trend strategies usually let winners run but cap any position, for example at twice its starting size.
• Full slots: decide in advance whether a new signal is skipped, or replaces the weakest holding, and how "weakest" is measured.
• Empty slots: cash, or a temporary index fund holding? Holding cash lowers returns in rising markets; holding an index fund keeps you invested but blurs the strategy's results. Pick one and record it.
• Risk per slot: check that a stop-out in every slot at once would not exceed your maximum drawdown limit.

Monitoring and the kill-switch

Track the portfolio every month against three things: your targets, an S&P 500 index fund, and the backtest's expectations.

• If a drawdown goes well beyond the worst in your backtest (for example 1.5 times the worst), stop opening new positions and review: is the market different, or are you executing differently?
• If live trades look different from test trades (different holding times, much bigger slippage), find out why before adding money.
• Do not change rules in the middle of a drawdown because of the drawdown. Change them, if at all, after a review, and record the change.

A real example to study

The DanioX stocks page runs its live signals as a follow-it portfolio with a fixed number of slots and reports it next to the S&P 500, and the stocks backtest shows how such slot portfolios behaved for each tested rule. Studying how those pages handle slots, exits and benchmarks is a useful way to sharpen your own rules. Check the pages for current results rather than any number you have seen elsewhere.

Next: the numbers behind surviving bad years, in drawdowns and risk of ruin.