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Since the launch of the S&P/ASX Index Series, the Australian capital market has evolved considerably. Instead of calling or faxing a stockbroker, market participants today can instantly trade stocks, ETFs, options and futures contracts with much shorter settlement times by tapping a screen in the palm of their hand. As financial markets develop, index methodologies must also evolve to ensure they continue to meet their objective and effectively represent the markets they intend to measure.

As stewards of the most widely followed Australian benchmarks, S&P Dow Jones Indices (S&P DJI), in partnership with the ASX, take a prudent approach by conducting extensive research on impact and incorporating feedback from a wide range of market participants before a decision is made to modify an index methodology.

Following a market consultation in July and August, the S&P/ASX index committee reviewed the feedback and approved three changes to the wider S&P/ASX Index series.

What Are the Changes?

Three modifications to the S&P/ASX Index Series have been approved and will be implemented in the upcoming September rebalance.

1. Reduce calculation period. Currently, the S&P/ASX 200 and S&P/ASX 300 consider companies for index inclusion based on a six-month average of their float-adjusted market capitalization and daily liquidity. This calculation period will be reduced to three months.
2. Apply two-way turnover. Currently, stocks are added to an index once an existing constituent has exited the index due to a sufficiently low rank below an exit buffer. Going forward, once a company ranks above an index entry buffer, it can be added regardless of the ranking of other existing index constituents.
3. Lower Investable Weight Factor (IWF). The IWF is the ratio of available free float shares to total shares outstanding. It reflects the proportion of shares available for trading in the market. The current minimum IWF for index inclusion is 0.3 and will be reduced to 0.15.

The approved changes will impact the S&P/ASX 300, S&P/ASX 200, S&P/ASX 100, S&P/ASX 50, S&P/ASX 20, S&P/ASX MidCap 50 and S&P/ASX Small Ordinaries, as well as any other index derived from these indices within the series, such as GICS^® sector indices.

Why Are These Changes Being Proposed?

Today’s market environment is faster paced with regard to information flow, trading execution, holding periods, settlement and other factors. Reducing the calculation period on index eligibility criteria to three months will allow the indices to react more quickly to market changes.

The introduction of a two-way buffer will also allow more timely adjustments to index constituents. A two-way buffer is also the more common approach used among other benchmarks operated by S&P DJI.

The current IWF of 0.3 is higher than most other benchmarks operated by S&P DJI and can prevent some companies being included in the index series despite meeting minimum liquidity requirements. Furthermore, the minimum free float threshold is higher than the ASX minimum float requirement of 20% at listing.

Impacts of Proposed Changes

Based on back-testing analysis of the approved changes, we anticipate faster-growing companies being added to and represented in indices earlier; conversely poorer-performing companies can exit more quickly.

Our modeled analysis, which was included in the market consultation, resulted in a modest performance benefit to the S&P/ASX 200 and S&P/ASX 300, while not materially affecting turnover or creating a “yo-yo effect” of companies entering and exiting indices within one or two rebalances.

Modeling of the three approved enhancements to the S&P/ASX Index Series resulted in a tracking error of less than 0.2% across all periods for the S&P/ASX 200 and S&P/ASX 300.

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Conclusion

The upcoming modifications to the S&P/ASX Index Series are intended to enhance the indices’ representativeness by allowing them to more quickly reflect changing market conditions while further aligning with S&P DJI index conventions.

The three index changes outlined in this paper will be implemented in conjunction with the September 2025 rebalancing, which is effective after the close on Friday, Sept. 19, 2025.

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