Costco's Stock Plummets 2.34% on $2.19 Billion Surge in Volume Ranks 35th in U.S. Dollar Turnover Amid Mixed Operational Signals

On September 10, 2025, Costco Wholesale (COST) closed down 2.34% despite a 49.92% surge in trading volume to $2.19 billion, ranking it 35th among U.S. equities by dollar turnover. The decline followed mixed signals from recent operational updates and sector positioning.

Recent reports highlighted Costco's ongoing struggle to balance membership growth with merchandise margin pressures. A key report noted that while the retailer added 1.2 million members in the latest quarter, average basket sizes declined slightly due to promotional strategies. Analysts observed that the stock's sensitivity to macroeconomic indicators intensified as investors reevaluated its defensive positioning amid rising interest rates.

Supply chain dynamics also played a role. A detailed analysis revealed that Costco's inventory management faced challenges from delayed supplier deliveries in key categories, though management emphasized that these were temporary. The company's focus on expanding its pharmacy and optical services continued to attract long-term investors, though short-term volatility remained elevated.

To run this back-test rigorously I need to pin down a few practical details about the universe and mechanics. Once these are settled I can pull the data and run the simulation. Universe: Do you want to consider all U.S. listed common stocks (NYSE + NASDAQ + AMEX)? Or would you prefer a standard universe such as the Russell 3000 or S&P 1500? Daily ranking metric: Use raw share volume (shares traded) or dollar volume (shares × close price)? Rank on the prior trading day’s volume and form the portfolio at today’s open (typical) – is that acceptable? Weighting & rebalancing: Equal-weight each of the 500 names, rebalancing every trading day after one-day holding period? Performance conventions: Time period: 2022-01-03 through today (2025-09-10). Return series expressed in USD, excess returns over risk-free, or simple total returns? If the defaults below work for you, just say “run with defaults”; otherwise let me know any changes.