Markets closed mixed, the S&P and Dow higher while the Nasdaq was lower after Fed Reserve chair Janet Yellen gave a mostly upbeat assessment of the economic outlook but warned on housing. The tech sell off continues with heavy falls in Groupon, AOL and Twitter after several companies disappointed with their results. Attention continues to focus on Alibaba, with concern its pending IPO could take further attention away from peers in the tech sector. Jane Searle reports from the NYSE.

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