Define Universe

Using today's index constituents for a historical backtest introduces survivorship bias - you only trade winners that stayed in the index, inflating returns by 1-2% per year.

The Problem

The S&P 500 today contains companies that survived and grew. The 2010 index contained firms since acquired, delisted, or bankrupt. A backtest on current members never sees these failures and overstates performance.

The Pattern

WRONG

import polars as pl