Jupiter Media Metrix (JMXI) was down 18.5% in midafternoon on the heels of its warning that it will lose 22 to 23 cents per share in the fourth quarter. The Internet research company's previous estimate was for a 1-cent profit. It also said it would lay off 80 employees. Deutsche Banc Alex. Brown, Morgan Stanley Dean Witter, Robertson Stephens and Thomas Weisel all piled in with downgrades.

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