Allocation drift

Allocation drift is the movement of actual weights away from target without any decision to move them. If equities rise 20% while bonds are flat, a 60/40 portfolio drifts to about 64/36. Income, contributions and withdrawals cause drift too.

Drift matters because ranges are tested on actual weights. A policy that caps equities at 62.5% would be breached by that market move alone. This is usually called a passive breach: no trade caused it, so the usual response is to rebalance within a reasonable period.

Measuring drift needs a current value for every holding and the class each belongs to. A missing price is never read as zero; the result is “not evaluated”. In Acclimetry, drift past a strategic band returns a breach under the asset allocation range template.