US Federal Interest Expense (Accrued, Public Issues)
Methodology
Treasury Fiscal Data 'Interest Expense on the Public Debt Outstanding', filtered to the accrued-interest-expense line for marketable public issues (the single largest interest expense category). Monthly, via the Bureau of the Fiscal Service. Genuinely volatile month-to-month: most marketable notes/bonds pay semiannual coupons on fixed February/August or May/November schedules, so accrued expense recurringly jumps 40-90% in coupon-heavy months (confirmed across 11 separate months in the live 2010-2026 history) — a structural artifact of the debt calendar, not an anomaly.