CloseBell — notes on historical options data https://closebell.io/blog/ Survivorship bias, split-adjusted strikes, end-of-day snapshot timing, and what a full option chain actually costs. en-us 2026-11-01T00:00:00Z Survivorship bias in options backtests: the ~35% you can't see https://closebell.io/blog/survivorship-bias/ https://closebell.io/blog/survivorship-bias/ 2026-11-01T00:00:00Z 1,573 names on 2020-03-16, 1,521 on 2026-10-30; 546 no longer trade. What that survivorship bias does to a backtest and how to test for it. How option strikes get adjusted for a 10:1 split (NVDA, 2024) https://closebell.io/blog/split-adjusted-strikes/ https://closebell.io/blog/split-adjusted-strikes/ 2026-11-01T00:00:00Z The 10.00 call re-listed at 1.00 and a fresh 10.00 series alongside it. The exact rows, the mapping rule, and why "adjusted" is a lie without a marker. End-of-day options: what "the 15:45 close" means — and what it doesn't https://closebell.io/blog/eod-snapshot-time/ https://closebell.io/blog/eod-snapshot-time/ 2026-11-01T00:00:00Z Our EOD quote is the last consolidated top of book at or before 16:00 ET, stamped 16:00:00. A mark, not a fill. What that costs a backtest. Where to get historical US options data in 2026 (and what it will cost) https://closebell.io/blog/free-historical-options-data/ https://closebell.io/blog/free-historical-options-data/ 2026-11-01T00:00:00Z DoltHub, a public Parquet mirror, Kaggle, OptionsDX and the paid vendors — compared on start date, fields, delisted coverage and size.