Shares in Sears Holdings Corp. dropped 8 percent Thursday after news that it will be replaced on the iconic S&P 500 by chemical company LyondellBasell Industries Sept. 4.Shares closed at $52.92, losing $4.55, or 7.92 percent. They hit a low of $52.77 earlier in the session.A drop in share price is typical when a company is removed from an index because fund managers have to sell that stock to ensure their holdings match the companies in the index.The Hoffman Estates-based retailer said Wednesday it was dropped because its public float -- the number of shares held by public investors -- has been "well below the 50 percent threshold for inclusion" for an "extended period of time," said Standard & Poors, which operates the S&P 500, in a statement.Recent regulatory filings indicate that Sears Chairman Edward Lampert holds roughly 62 percent of the company."While we're disappointed in Standard & Poor's decision, we would point out that the action is rules based and solely a function of the public float of our shares, and not the valuation or performance of the company," said Sears spokesman Chris Brathwaite in a statement."We would also highlight that S&P recently boosted Sears Holdings' credit ratings outlook to stable from negative, saying the company had improved its liquidity through our financial and operational discipline."crshropshire@tribune.com | Twitter: @corilyns 
  
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 Sears Holdings, a founding member of the Standard & Poor's 500-stock index, is getting booted from the venerable benchmark—and its own stock is to blame. 
  
 Once America's most iconic retailer, Sears will lose its place in the market measure because its "public float," or the number of shares that are in the hands of public investors, has hovered below a key threshold, according to a release issued Wednesday by S&P; Dow Jones Indices. 
  
 But the S&P; on Thursday added that recent news of a planned rights issue was the ... |||||

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– Observers of the S&P 500 might notice the absence of a familiar name starting Wednesday: Sears is being dropped from the index, after being a part of it since the S&P's creation in 1957, reports CNNMoney. One of the main reasons cited for the move is a bit technical, explains the Wall Street Journal: The number of Sears shares held by public investors, or the "public float," has fallen below the necessary 50% threshold for inclusion. Shares of Sears Holding Company—parent of the Sears, K-Mart, and Land's End chains— were down 8% today on the news, notes the Chicago Tribune. That's typical because mutual funds that mimic the S&P have to sell their Sears stock and buy up that of its replacement on the index. And that company is ... Dutch chemical firm LyondellBasell Industries.