The National Stock Exchange and the Securities and Exchange Board of India clarified new closing auction rules after the Nifty and Sensex indices diverged.
This discrepancy occurred during the first application of a new pricing mechanism, creating volatility and confusion among market participants as the two primary Indian indices moved at different speeds.
Under the new SEBI guidelines, the previous method of calculating a 30-minute average price has been replaced by a single-price auction [3]. This shift was intended to increase transparency and efficiency in determining the final closing price of equities. However, the transition led to a visible gap between the indices on the day of implementation.
The Nifty index rose 1.6% [1], while the Sensex index gained only 0.7% [2]. Market observers noted that Nifty futures were trading below the spot index during the period of instability.
Sharp market movements were observed after 3:15 PM IST [4]. The surge in the Nifty while the Sensex lagged prompted the National Stock Exchange to issue a clarification to explain the mechanics of the Closing Auction Session (CAS).
The NSE said the new rule was designed to provide a more accurate closing price by consolidating orders into a single auction. The gap between the indices reflected how different participants reacted to the new auction-based system compared to the old averaging method, a change that shifted the timing and nature of liquidity at the end of the trading day.
“The Nifty index rose 1.6%, while the Sensex index gained only 0.7%.”
The divergence between the Nifty and Sensex highlights the sensitivity of high-frequency trading and index arbitrage to structural rule changes. By moving from a time-weighted average to a single-point auction, SEBI has changed the incentive structure for traders in the final minutes of the session, which can lead to temporary pricing anomalies until the market adapts to the new liquidity patterns.
