Calendar vs. Threshold Rebalancing: What's the Difference?
There are broadly two ways to decide 'when' to rebalance: watch the calendar, or watch how far the weights have drifted.
Calendar Rebalancing
Calendar rebalancing returns weights to their targets unconditionally at a set interval. E.g., every January, or every half-year.
The advantage is that it is simple and strongly disciplined. Because you act whenever the date arrives, there is no need to agonize over 'should I do it now.' The drawback is that even when the market has barely moved, you may trade unnecessarily just because the date has come, incurring costs.
Threshold Rebalancing
Threshold rebalancing adjusts only when a weight has drifted beyond a set range from the target. E.g., act when an asset drifts ±5%p from its target.
The advantage is that you trade only when needed, saving costs and responding more sensitively to large imbalances. The drawback is that you must check weights frequently, and trades can cluster in a highly volatile market.
Combining the Two & Comparing Performance
In practice, a hybrid of the two is common. You 'check at a set interval, but only actually adjust when the drift exceeds the threshold.' It is a compromise: check regularly, trade only when needed.
According to Vanguard's research, the difference in risk and return between calendar and threshold approaches was not large. What matters is not 'which method is correct' but 'whether you pick one and stick to it without emotion.'
For an account with high trading costs and taxes, the threshold (only-when-needed) approach may be advantageous; if you want simplicity, the calendar approach is convenient.
よくある質問
Q. Which method gives higher returns?
Studies including Vanguard's find that the long-term difference in risk and return between the two methods is small. Sticking to a rule consistently affects the outcome more than the choice of method.
Q. What happens if I never rebalance at all?
The weight of the asset that has risen keeps growing, and the portfolio gradually becomes concentrated in that asset. As a result, it can end up far more aggressive than the risk level you originally set, and you may suffer a larger-than-expected drawdown in a big downturn.
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