Intraday Price Formation in Us Equity Index Markets

Abstract

The market for US equity indexes has traditionally comprised floor-traded index futures contracts and the individual markets for the component stocks. This picture has been altered by the advent of exchange-traded funds (ETFs) that mirror the indexes, electronically-traded, small-denomination ("E-mini") futures contracts, and (for the S&P 500) a family of sector ETFs that break the index into nine components. This paper empirically investigates price discovery (price leadership) in this new environment. The specifications are estimated at very fine (up to one second) time resolution. The principal findings are as follows.

- For the S&P 500 and Nasdaq-100 indexes, the index market comprises an ETF, a regular floor-traded futures contract and an E-mini futures contract. The paper finds that most of the price discovery for both indexes occurs in the E-mini markets: price changes in the E-mini futures prices generally lead those in the regular futures contracts and the ETFs.

- The market in the S&P 400 MidCap index comprises only the ETF and the regular futures contract. Both securities contribute substantially to price discovery, but the ETF appears to dominate.

- The sector ETFs can closely replicate the S&P 500 ETF. Nevertheless, trading activity across the sector ETFs varies considerably. The most actively traded sector (technology) contributes a modest amount to price discovery in the overall index. The other sector ETFs play only a minor role.

Keywords: Equity, index futures, exchange traded-funds, electronic markets, price discovery, price formation, SPDR

JEL Classification: G100, G290