Survivorship Bias

Testing a strategy only on securities that exist today removes the worst performers from history, inflating backtest returns by 1-2% per year.

The Problem

If you download today's S&P 500 constituents and run a backtest starting in 2008, you exclude Lehman Brothers, Bear Stearns, Washington Mutual, and every other company that was removed after distress. The remaining panel has a built-in upward bias because you already know these firms survived.

This is worst for value and small-cap strategies, which overweight distressed names - exactly the ones that get delisted. A long-short value backtest on a survivor-biased universe can show +3% alpha that vanishes entirely on a survivorship-free dataset.

The Pattern

WRONG

import polars as pl

# Use today's index members for a historical backtest
current_members = pl.read_csv("sp500_current.csv")  # 2024 list
prices = pl.read_parquet("prices.parquet")

backtest_universe = prices.filter(
    pl.col("symbol").is_in(current_members["symbol"])
)
# Missing: every company removed between 2008 and 2024

CORRECT

import polars as pl

# Use point-in-time index constituents
constituents = pl.read_parquet("sp500_constituents_history.parquet")
prices = pl.read_parquet("prices.parquet")  # includes delisted symbols

# For each date, use only the members as of that date
backtest_universe = prices.join(
    constituents,
    on=["symbol", "timestamp"],
    how="inner",
)

Delisting Returns

Dropping a delisted stock on its last trading day ignores the terminal return. Include delisting outcomes:

delisting_return = {
    "bankruptcy":      -1.00,   # total loss
    "acquisition":      0.00,   # use actual tender premium if available
    "going_private":    0.00,   # use tender offer price
    "exchange_change":  0.00,   # continue tracking on new exchange
}
# Apply the delisting return on the last traded date

Data Source Quality

Source	Survivorship-free?	Notes
CRSP	Yes	Gold standard, includes delistings
NASDAQ Data Link (Wiki)	Yes	1962-2018, includes delisted companies
Yahoo Finance	No	Current tickers only
Most free APIs	No	Survivor-biased by default
Crypto exchanges	Partial	Coins get delisted frequently

Guardrails

• Any universe built from a single "current members" list is survivor-biased.
• S&P 500 changes 20-25 constituents per year; over a 10-year backtest that is 200+ changes.
• Free data almost always has survivorship bias. Budget for CRSP or equivalent if equity research is serious.
• ETF and crypto markets have high turnover - fund closures and coin delistings are common and material.

Production Implementation

ml4t-data exposes a survivorship-bias-free historical US equities archive through 2018:

from ml4t.data.providers.wiki_prices import WikiPricesProvider

provider = WikiPricesProvider()
aapl = provider.fetch_ohlcv("AAPL", "2010-01-01", "2018-03-27")
# The archive includes delisted companies; PIT constituents still need explicit handling

Checklist

• Universe uses point-in-time index constituents, not current membership
• Delisting returns included (not silently dropped)
• Index reconstitution events tracked over the backtest period
• Data source documented for survivorship treatment
• Value/small-cap strategies double-checked for survivorship sensitivity