Signal age is the elapsed time since an interaction signal occurred. Time-decay weighting reduces older signals’ influence while preserving residual value for longer-lived documents. section 8.5.2
starting_weight × 0.5(signal_age / half_life)
Calculate a signal’s time-weighted contribution as starting weight × 0.5^(signal age ÷ half-life). section 8.5.2
A 30-day half-life assigns approximately:
- 100% weight to a current signal, 75% to a 15-day-old signal, and 50% to a 30-day-old signal. section 8.5.2
- 25% weight to a 60-day-old signal and 12.5% to a 90-day-old signal. section 8.5.2
Applying signal age in aggregation
A signals query can group the latest click for each user, query, and document, apply a 30-day half-life with POW(0.5, signal_age / 30), sum boosts by query and document, and order results by boost. section 8.5.2
