ml4t-survivorship-bias skillA
ml4t-survivorship-bias is agent-read markdown (skill) from ml4t/skills: Account for delisted and removed securities in historical analysis. Use when constructing universes or computing cross-sectional features to avoid survivor-only inflation..
Indexed from public GitHub and served as immutable, content-addressed versions. Install it pinned to an exact SHA-256 with the mdr CLI, and every file is verified against the hash recorded here before it reaches your agent. The deterministic audit below grades the latest version, and the same file always earns the same grade.
What the file says
# 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
```python
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
```python
import polars as pl
# Use point-in-time index constituents
…Read the whole file at its exact version.
How to install
mdr add ml4t/skills/ml4t-survivorship-bias@git:20260901.c415df0mdr add ml4t/skills/ml4t-survivorship-bias@sha256:7ad64d3ed656fabdPin to a label to follow the author's releases, or to a sha256 to freeze the exact bytes forever. Either way the resolved hash is written to mdr.lock, and mdr install reproduces it on any machine.
[](https://markdownregistry.com/a/art_xz4vpd2iu7bcmyk2)
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Versions
| version | committed | commit | size | audit | |
|---|---|---|---|---|---|
| git:20260901.c415df0 latest | 2026-09-01 | c415df0 | 3,891 B | A | view · diff |
| git:20260528.303089e | 2026-05-28 | 303089e | 3,896 B | A | view |
Audit of the latest version
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- pass: Frontmatter declares a name
- pass: Frontmatter declares a description
- pass: Size between 200 bytes and 200 KB (3891 bytes)
- pass: No zero-width or bidi control characters
- pass: No instruction hidden inside an HTML comment
- pass: No link to an exfiltration or paste host
- pass: No credential-shaped string
- pass: No instruction to send local credentials anywhere
- pass: No text hidden with inline styles
- pass: No prompt-injection phrasing
- pass: No curl or wget piped into a shell
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Source
ml4t/skills · 19 stars · license Apache-2.0 · pushed 2026-09-24 · branch main
API
GET https://markdownregistry.com/api/v1/artifacts/art_xz4vpd2iu7bcmyk2 GET https://markdownregistry.com/api/v1/resolve?ref=ml4t/skills/ml4t-survivorship-bias GET https://markdownregistry.com/api/v1/blob/7ad64d3ed656fabd6de43ada3a5454dfed088e5a8c312720a985ac1ddae78b78
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