What are the differences between calendar rebalancing and percentage-of-portfolio rebalancing, and which approach is better for different investor types?
I'm studying CFA Level III and trying to understand the trade-offs between rebalancing on a fixed schedule versus rebalancing when allocations drift beyond a threshold. What are the pros and cons of each, and does the optimal choice depend on the investor?
Rebalancing discipline is essential for maintaining a portfolio's risk profile over time. The two primary approaches -- calendar-based and percentage-of-portfolio (threshold-based) -- differ in their trigger mechanisms, transaction costs, and ability to control risk.
Calendar Rebalancing:
Rebalance at fixed time intervals (monthly, quarterly, annually) regardless of how far allocations have drifted.
Percentage-of-Portfolio (Threshold) Rebalancing:
Rebalance only when any asset class weight deviates from its target by more than a specified threshold (e.g., +/- 5% of target weight or +/- 3 percentage points).
Comparison:
| Criterion | Calendar | Percentage-of-Portfolio |
|---|---|---|
| Trigger | Fixed dates | Allocation drift |
| Monitoring | Low (check at intervals) | High (continuous or daily) |
| Transaction costs | Predictable | Variable; lower in calm markets |
| Risk control | May allow large drift between dates | Tight; bounds drift precisely |
| Trending markets | Rebalances even when drift is small (unnecessary cost) | Defers action when drift is small |
| Volatile markets | May rebalance too infrequently | Triggers frequently; higher costs |
| Implementation | Simple; calendar-based | Requires monitoring infrastructure |
Worked Example:
Forestdale Foundation targets 60% equity / 40% fixed income on a 268M) and fixed income to 33% (28M equity, buy 12.4M equity at that point. Drift never exceeded the band.
The threshold approach caught the drift 5 weeks earlier, reducing the period of unintended risk.
Hybrid Approach:
Many institutional investors combine both: review the portfolio on a fixed schedule (quarterly) but also monitor for threshold breaches between review dates. This captures the simplicity of calendar-based governance with the risk control of threshold triggers.
Investor-Specific Recommendations:
| Investor Type | Recommended Approach | Reason |
|---|---|---|
| Large pension fund | Threshold + calendar governance | Needs tight risk control; has monitoring infrastructure |
| Individual investor | Calendar (quarterly) | Simpler; lower monitoring burden |
| Taxable account | Calendar (annual) | Minimizes realization events |
| Endowment | Threshold with wide bands | Lower turnover; long horizon tolerates drift |
| Insurance company | Tight threshold | Regulatory constraints require precise ALM |
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