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According to Bloomberg, economists estimated that the index increased to 90.6.
The survey found that consumers believed that the economy would continue to improve modestly, but weren't as bullish about their income prospects and buying plans.
"Buying plans suffered from the perception that no additional price discounts would be offered," said Richard Curtin, the survey's chief economist, in the release.
The final August index unexpectedly fell, missing economists' expectations, as young people expressed worries about their personal finances.