ml4t-survivorship-bias SKILL.md skillA
ml4t-survivorship-bias from ml4t/skills is an agent skill: a SKILL.md folder an AI agent loads when a task calls for it.
mdr add ml4t/skills/ml4t-survivorship-bias@git:20260901.c415df0Pins git:20260901.c415df0 in mdr.lock. Pin the fingerprint instead.
- Audit grade
- A, 17 of 17 checks passed
- Kind
- SKILL.md skill
- Repository
- ml4t/skills
- Path
concepts/survivorship-bias/SKILL.md- Versions recorded
- 2
- Latest version
- git:20260901.c415df0, committed Sep 1, 2026
- First version recorded
- May 28, 2026
- Size
- 3,891 bytes
- Repository stars
- 41
- License
- Apache-2.0
Account for delisted and removed securities in historical analysis. Use when constructing universes or computing cross-sectional features to avoid survivor-only inflation.
Collected from public GitHub. Every version is saved exactly as it was and never changes. Install it at an exact version with the mdr tool, and every file is checked before it reaches your agent. The main file is checked against the SHA-256 fingerprint recorded here, and the others against the fingerprints GitHub keeps for its source commit. The automatic checks below grade the latest version, and the same checks always give the same file 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@sha256:7ad64d3ed656fabdThe line at the top of this page pins the label git:20260901.c415df0, which is what most people want. This one pins the sha256 fingerprint of the contents instead, so it survives a label being moved. Either way the exact version is written to mdr.lock, and mdr install downloads and checks those contents again, so it installs them exactly or stops with an error.
[](https://markdownregistry.com/a/art_xz4vpd2iu7bcmyk2)
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Versions
The registry has recorded 2 versions of this file, between May 28, 2026 and Sep 1, 2026, about one every 96 days.
| 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
- pass: Frontmatter block present
- 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
- pass: No recursive delete of root, home or parent
- pass: No instruction to read or print local credentials
- pass: No base64 blob over 200 characters
- pass: No link to a raw IP address
- pass: No script tag
Source
ml4t/skills · 41 stars · license Apache-2.0 · pushed 2026-11-08 · 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
Your AI assistant does the legwork. You hear about the deals worth your word. Hand yours the standing instructions at modelranch.com and it joins the network that reads files like this one.
Guides: the SKILL.md format and its required fields · what to check before you install a skill · how to pin a skill to an exact version · where to find agent skills · keeping a team on the same skills · which agent file does what.