BIIB·Biogen Inc.

Realized volatility (21d)

22.4% 23rd pct

Below its peak

-47.1%

vs 200-day average

+16.5%

Up days (1y)

53%

Biogen Inc. is currently running 22.4% annualised volatility over the last 21 sessions. Measured against BIIB's own 13 years of daily returns rather than a market-wide average, that is the 23rd percentile — unusually subdued by its own history, against a typical reading of 29.1%. The distinction matters because a number that looks calm for one stock is turbulent for another, and the absolute figure on its own tells you neither.

BIIB is trading -47.1% below its running peak. Across 12 full years on file, its median worst-drawdown-in-a-year was -31.0%, and the deepest was -46.2% in 2021. The current drawdown is already deeper than this stock's typical annual low point, which puts it in the part of its range where past recoveries began — and where past declines also continued.

Price sits +16.5% from its 200-day moving average. Against every other day in its history, that gap ranks at the 85th percentile — a wider gap than this stock usually carries. Over the past year BIIB closed higher on 53% of sessions, and it is currently 1 session into a rising streak.

On October specifically: over 14 years of records, BIIB finished the month higher 5 of 14 times, with a median return of -1.7% and an average of -1.1%. Its strongest month historically has been November at +2.1% on average, its weakest March at -4.7%. Seasonality is a weak signal on its own — a decade is a small sample and regimes change — but it is a real distribution rather than a rule of thumb.

Overnight risk is measurable too: BIIB has opened more than 2% away from the prior close in 109 of 3,267 sessions (3.3% of the time), with the largest gaps running +5.7% and -30.7%. For anyone holding through an event, that rate is a better guide to overnight exposure than the intraday range.

Across the last 3 dated reports on file, BIIB moved an average of 3.1% in the session after earnings, closing higher 0 of those 3 times, with a median move of -3.6% and a largest of -3.8%.

All figures computed from 3,269 daily closes between 2013-10-15 and 2026-10-14, split-adjusted, recomputed daily. Percentiles are against BIIB's own 13-year history, not a peer group or index.