Analysis · Finance

A Rebound Is a Difference, Not a Verdict

By AWEI · AI-compiled · Published 2026-09-18 · Analysis prepared 2026-09-18 · 9 sources · doi.org, doi.org, doi.org

September’s equity advance was a measured return against a chosen baseline, not a direct reading of policy. Research on expectations and event windows shows why.

A September rebound looks like a single event. In the reporting, it is several different measurements placed side by side: an intraday S&P 500 quote, a closing level, a fund-flow total from the prior week, and a futures-implied probability of a rate increase [4][5][6]. The word “rebound” does not make them one object.

The price is a subtraction

A rebound is not a property of the market. It is a difference between two prices, with the baseline chosen by the writer. Yonhap reported the S&P 500 at 7,664.52, up 0.96%, at 9:44 a.m. New York time on September 11, while FISCO’s wrap reported a September 11 close of 7,656.98, up 65.28 points [6][8]. Both describe an advance, but they are not the same moment. Al Bayan supplied an intraday quotation of 7,636.75 [5]. The numbers can coexist because they are returns over different windows, not competing facts about one price.

Event studies make the same point formally. Bauer, Offner, and Rudebusch calculate abnormal returns as the difference between observed raw returns and predicted returns from a market model estimated before the event [3]. Their one-day and three-day windows around the Inflation Reduction Act show that the size and even the sign of a response can depend on the benchmark and the window. A rebound, in this sense, is a residual: what remains after a model of normal movement is subtracted.

Expectations are not the same as prices

A second distinction runs through the September reporting. The consumer price index is a measured price change. The probability of a Federal Reserve rate increase is an inference from futures prices. A Reuters survey of economists is a set of stated forecasts. These are different objects, and the research evidence shows why they cannot be averaged into one “market expectation.”

In a randomized controlled trial with U.S. households, Coibion, Gorodnichenko, and Weber found that simply telling people the Fed’s inflation target reduced their average inflation forecast by about one percentage point relative to a control group, while a news article covering the same FOMC meeting had an effect about half as large [2]. The treatment changed stated expectations, not observed prices. The effect also faded: it had largely dissipated after three months and was gone by six months [2]. This is a study of how information changes survey responses, not a direct observation of trading.

Pham, Nguyen, and Ho develop a psychological inflation index for Vietnam that weights price changes by purchasing frequency and loss aversion [1]. Their index is highly correlated with headline inflation, with a reported correlation of 0.98, and cointegrated with it [1]. But the index is a constructed perception measure, not a market price. The authors state that the measurement is empirically checked only in Vietnam [1]. It cannot be transferred to a U.S. equity session as evidence of what investors perceived.

The graph has more than one edge

A market explanation often looks like a line from one cause to one effect: cheaper oil, higher stocks. The September reports contain at least two candidate edges. Yonhap reported October WTI futures down 3.49% at $98.90 a barrel, and both Yonhap and FISCO attributed support for stocks to cheaper oil [6][8]. At the same time, the core CPI surprise increased the market-implied probability of a rate increase [6]. These are competing channels, not a single path.

The event-study evidence shows how heterogeneous such channels can be. Around the IRA’s green event, utilities and construction rose while oil and gas and mining fell [3]. The same policy news produced opposite signs across industries. That does not prove which channel dominated on September 11. It shows that a single headline can travel along multiple edges, and the net index move is the sum of those paths, not a vote on one explanation.

What the rebound cannot explain

The September reports also contain a boundary. Al Bayan cited LSEG Lipper data showing $32.27 billion in net U.S. equity-fund withdrawals during the week ending September 9 [5]. That flow occurred before the rebound. It cannot be read as money leaving during the advance, and even a matching window would describe fund subscriptions and redemptions, not the experience of every holder. A recovered price does not restore a position already closed.

The research carries the same limit. The randomized trial measured forecast revisions, not trades [2]. The psychological inflation index was built from Vietnamese survey and price data, not U.S. market data [1]. The IRA event study identified reactions to two specific policy announcements, not a general model of any inflation surprise [3]. None of these sources directly studied the September 11 session. They supply a grammar for distinguishing price changes, expectations, and forecasts; they do not supply the missing causal sentence.

A competing explanation appears in the reporting itself: Yonhap noted that markets may have already prepared for the start of a rate-increase cycle [6]. That is a positioning account, not a measured contribution of oil or inflation to the session. The news reports are contemporaneous institutional accounts, not peer-reviewed measurements; the research papers are a randomized trial, an index construction, and an event study, not cohort studies or age-based comparisons of the September rebound.

The intraday quote was 7,664.52; the close was 7,656.98. The rest is interpretation.

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