What spending policy formulas do endowments use and how do they trade off stability vs responsiveness?

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Endowment spending formulas translate asset values into annual payouts — simple rate, moving-average, or Yale-rule hybrid.

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**1. Simple spending rate (bank-rate method)**: Spending = rate × current market value. Highly responsive to market swings — operating budgets whiplash in volatile markets. Rarely used in isolation.

**2. Moving-average method**: Spending = rate × average of last N years' market values (typically 3 or 5 years). Smooths volatility but lags market moves. Most common for US university endowments.

**3. Hybrid/Yale rule**: weighted blend. Spending_t = α × (prior_year_spending × (1 + inflation)) + (1 − α) × (target_rate × MV_{t−1}). α typically 0.70-0.80. Heavily weights prior spending for stability, while gradually correcting toward target rate.

Example: Millfield College uses Yale rule with α = 0.80, target rate 5.00%. Prior year spending $28M, inflation 3.2%, prior MV $620M.
Inflation-adjusted prior: $28M × 1.032 = $28.9M
Market-based: 5.00% × $620M = $31.0M
Blended: 0.80 × $28.9M + 0.20 × $31.0M = $29.32M

The Yale rule dominates large endowments because it preserves budget stability year-over-year while slowly correcting drift. α = 0.70-0.80 means full adjustment to a new market level takes roughly 4-7 years. Pure moving averages with long windows can create 'phantom' wealth effects during prolonged bear markets.

What spending policy formulas do endowments use and how do they trade off stability vs responsiveness?

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