Observation Shift

Observation Shift

Observation shift is a variant of the lookback period convention in which both the overnight rate and the day-weight (the number of calendar days each rate covers) are taken from the observation date — which is the lookback period earlier than the accrual date.

Observation Shift vs Lookback Without Shift

Feature	Lookback Without Shift	Observation Shift
Rate shifted?	Yes (5 BD back)	Yes (5 BD back)
Day weight shifted?	No (stays on accrual date)	Yes (taken from observation date)
Standard for	Syndicated loans (ARRC)	FRNs, bonds, some bilateral loans
Annualisation days	Accrual period days	Observation period days

Why It Matters

Because weekend and holiday day weights depend on where weekends and holidays fall in the calendar, the observation period and accrual period may have different patterns. This means the total calendar days counted under observation shift (the observation period days) may differ from the actual loan period days.

Under observation shift, the correct annualisation denominator is the sum of observation period day-weights, not the accrual period calendar days. Using the wrong denominator inflates or deflates the effective rate.

Example

For a June 1 – June 30 accrual period with a 5-business-day lookback and observation shift:

• Rates and day weights are taken from approximately May 22 – June 23
• The observation period has different weekend structures than the accrual period
• Annualisation uses the observation period's total days (~30), not June's 29 days

When to Use Observation Shift

Observation shift is the standard for floating-rate notes (FRNs) and bonds referencing SOFR, SONIA, or €STR. It is also used in some bilateral bank loan agreements. The loan documentation will specify which convention applies.

Compare observation shift vs lookback without shift side-by-side using the RFR Loan Calculator.