# Survivorship bias: testing on today's NIFTY 50 adds 3.4% a year that never existed

> The same simple portfolio on the real NIFTY 50, stock list as it was on each day, versus today's 50 stocks back-tested to 2016. Today's list turns ₹1 lakh into ₹4.9 lakh instead of ₹3.6 lakh, because it quietly picks the winners in advance.

- Published: 2026-10-07
- URL: https://datapointx.com/research/in/survivorship-bias/
- Tags: survivorship-bias, method, data
- Verdict: Today's 50 stocks returned 16.2% a year since 2016; the real index stocks returned 12.8%.
- Source: DataPointX Research (education and research only, not investment advice)

Most backtesting tools and tutorials start the same way: download the stocks in the NIFTY 50 **today**, and test
a strategy on them over the last ten years. It sounds harmless. It isn't, because today's list is a list of
survivors. Stocks get into the NIFTY 50 by rising; they leave it by falling. A backtest on today's 50 has been told
the winners in advance.

To measure how much that matters, we ran the simplest possible portfolio on two versions of the index, from
January 2016 to August 2026: an equal-weight buy-and-hold of every stock in the universe, rebalanced daily.

- **The real NIFTY 50:** on each day, only the stocks that were in the index that day.
- **Today's NIFTY 50:** the 50 current members, back-tested over the whole period (each from the
  day our data starts for it).

₹1 lakh grows to about ₹3.6 lakh on the real index stocks, but to
₹4.9 lakh on today's list. The gap opens in almost every year, and the biggest gaps come in
years when the eventual winners were still small: in 2018 the real index stocks lost 6.1% while today's list made
1.2%, and in 2019 the gap was 8 points (3.0% against 11.0%).

## Where the phantom return comes from

Stocks enter the index after they've already gone up. Here are the current members that joined after 2016, and how
much each rose in the two years **before** it was added. A backtest on today's list owns all of that rise; the real
index owned none of it.

| Universe | CAGR | Total return |
|---|---:|---:|
| Real NIFTY 50 (point-in-time) | 12.8% | 261% |
| Today's 50 stocks, back-tested | 16.2% | 395% |
| NIFTY 50 index (price) | 11.2% | |

| Stock | Joined | Price change in the 2 years before joining |
|---|---|---:|
| TRENT | 2024-09-30 | +159% |
| BAJFINANCE | 2017-09-29 | +111% |
| ADANIENT | 2022-09-30 | +101% |
| BEL | 2024-09-30 | +58% |
| JSWSTEEL | 2018-09-28 | +52% |
| NESTLEIND | 2019-09-27 | +26% |
| INDIGO | 2025-09-30 | +21% |
| HDFCLIFE | 2020-07-31 | +1% |
| MAXHEALTH | 2025-09-30 | −3% |
| SBILIFE | 2020-09-25 | −19% |

| Year | Real NIFTY 50 | Today's 50 |
|---|---:|---:|
| 2016 | 8.7% | 10.7% |
| 2017 | 26.9% | 31.3% |
| 2018 | −6.1% | 1.2% |
| 2019 | 3.0% | 11.0% |
| 2020 | 19.4% | 22.9% |
| 2021 | 33.9% | 37.7% |
| 2022 | 6.1% | 8.3% |
| 2023 | 29.6% | 29.9% |
| 2024 | 10.2% | 15.2% |
| 2025 | 13.3% | 12.2% |
| 2026 | −1.5% | −1.5% |

The other half is the stocks that are missing. 29 stocks were in the index at some point since 2016
and aren't today, including YES Bank, Zee Entertainment, Vedanta, IndusInd Bank and BPCL. Several left after large
falls. A backtest on today's list never holds them, so it never takes those losses.

## A bonus finding: the gains came overnight

The same data shows something most intraday traders don't expect. On the real index, the average stock **fell**
8.1 bps between the open and the close of a typical day, even though the
stocks gained 12.8% a year overall. The decade's gains came between one day's close and the next
day's open. Buying at the open and selling at the close, every day, would have lost money before any costs.

**Limits of this test.**

Prices are back-adjusted for splits and bonuses but exclude dividends. Three former members have no minute data in
our snapshot (HDFC Ltd, Cairn India, Tata Motors DVR), so the real-index portfolio is one to three stocks short on
some days. Two of them left through mergers (HDFC Ltd into HDFC Bank, Cairn India into Vedanta), so their effect
on the comparison is likely small.

## What to take from this

- **Always test on the stocks that were in the universe on each day.** A "today's list" backtest adds about 3.4% a
  year here, before any strategy is involved.
- **The bias is biggest for long holding periods and momentum ideas,** because the winners are exactly the stocks
  that rose before joining.
- **Every DataPointX result uses point-in-time membership.** It's built into [dpx_bt](/docs/) and can't be switched
  off.