Time-Phased Safety Stock

Unlike a static safety stock number, TPSS calculates a distinct buffer quantity for each future time bucket (daily, weekly, monthly). This recognizes that the forecast error and supply variability for a period three months out is significantly different from next week.

• Mechanism: Applies a unique standard deviation of forecast error to each period.
• Result: A time-series of safety stock values, not a single constant.

The core logic matches buffer size to the projected demand volatility for each specific period. Periods with known promotions, seasonal peaks, or high forecast uncertainty receive a proportionally larger buffer.

• Example: A period covering Black Friday receives a 3x larger buffer than a standard week in February.
• Key Metric: Uses Coefficient of Variation (CV) calculated per time bucket.

TPSS models lead time variability as a time-phased risk. If a supplier's historical performance shows a 40% chance of a 2-day delay in a specific month, the buffer for the period covering that receipt is inflated accordingly.

• Data Input: Requires a lead time distribution per supplier, not just an average.
• Outcome: Prevents stockouts caused by predictable supplier performance dips during their own peak seasons.

The safety stock for a given period must cover the cumulative forecast error over the entire risk period, which equals the review period plus the lead time. TPSS sums the period-specific variances over this protection interval.

• Formula Concept: SS_t = Z * sqrt( Σ σ²_i ) for all periods i within the protection interval ending at t.
• Advantage: Correctly accounts for the compounding of uncertainty over longer horizons.

TPSS is not a set-and-forget calculation. It requires a continuous recalculation engine that ingests real-time demand signals and updates the entire forward-looking buffer profile as new data arrives.

• Trigger: Recalculated nightly or upon significant demand signal changes.
• Integration: Feeds directly into Dynamic Reorder Points and MRP systems to shift planned orders.

TPSS enables service differentiation at a temporal level. A company can target a 99% Cycle Service Level during a critical product launch month and automatically reduce the target to 95% for the same item during a low-priority period.

• Business Rule: IF month = 'Launch' THEN Z-factor = 2.33 ELSE Z-factor = 1.65
• Benefit: Optimizes inventory investment by not over-protecting low-criticality periods.