Survivorship Bias

A friend forwarded me a backtest last week. Beautiful curve. A basket of fifty large caps, run over ten years, and the line never had a lost decade. His question was simple. Why not just buy this.

So I asked which fifty names. They were today's fifty. And the market he was testing them against was the last ten years. That is the whole problem, and it has a name.

An index is not a fixed list. It gets reviewed twice a year, and names get swapped out. Roughly twenty five large caps cycled through the list over the last decade. Here is the part that matters: a name usually leaves the index after it has already shrunk or de-rated. It leaves after it has already hurt the people who held it.

Now run your backtest on today's fifty names. You are quietly holding only the companies that survived all the way to today. The ones that got deleted, often after halving, never show up. Not because the test is dishonest, but because they are not on the list you copied off the screen.

Same ten years, two different universes. The survivor list flatters the curve by something like a couple of percentage points a year. That gap is not skill. It is the losses you edited out.

The deletions are not noise. They are the data. They are the exact drawdowns you would have lived through if you had actually been in the market the whole time. Cut them and you are not measuring a strategy. You are measuring hindsight.

So before you trust any backtest, ask one question. Did it use today's members, or the index as it actually stood on each date. And does the strategy say what it buys and sells when a name falls out, or does it just assume that name was never there.

Every backtest is a story told by the survivors. The honest one keeps a seat for the names that did not make it.

How systematic investing actually works, in plain English: rupeecase.com/learn/

(Illustrative figures, educational only. Not investment advice.)