* GDP, initial claims data due
* Twitter shares to debut on NYSE
Qualcomm falls after results, outlook
* Futures off: Dow 10 pts, S&P 2.4 pts, Nasdaq 5.25 pts
By Chuck Mikolajczak
NEW YORK, Nov 7 (Reuters) - U.S. stock index futures slipped on Thursday, ahead of data on economic growth that may provide some clues as to how long the Federal Reserve will maintain its monetary stimulus measures.
* Investors will also brace for the market debut of Twitter Inc, which priced its initial public offering, above the expected range, at $26 per share to raise at least $1.8 billion, a sign of strong investor demand for the most highly anticipated U.S. public float since Facebook Inc.
* Data on tap for Thursday includes weekly initial jobless claims and the advance estimate of third-quarter gross domestic product at 8:30 a.m. (1330 GMT). Economists in a Reuters survey forecast a total of 335,000 new claims compared with 340,000 in the prior week. GDP is expected to show a 2.0 percent annualized pace of growth compared with a 2.5 percent rate in the final (third) Q2 estimate.
* The GDP data, coupled with Friday's payrolls report may give investors insight as to how log the Fed may keep its plan of $85 billion a month in bond purchases intact. The central bank's stimulus measures have been a key component of the 24.1 percent yearly gain in the S&P 500, putting the index on pace for its best yearly performance since 2003.
* S&P 500 futures fell 2.4 points and were below fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures fell 10 points and Nasdaq 100 futures lost 5.25 points.
* Qualcomm Inc shares stumbled 3.9 percent to $67 before the opening bell after the company reported quarterly earnings and forecast revenue below expectations.
* S&P 500 companies scheduled to report earnings on Thursday include Dow component Walt Disney Co, Nvidia Corp and Priceline.com Inc.
* According to Thomson Reuters data, of the 423 companies in the S&P 500 that have posted quarterly earnings through Wednesday morning, 68.3 percent have topped Wall Street expectations, above the 63 percent average since 1994 and the 66 percent beat rate for the past four quarters.
* Corporate revenue has been lackluster, however, with 53.5 percent of companies beating expectations, below the 61 percent rate since 2002 and slightly above the 49 percent rate for the past four quarters.
* European shares were marginally lower as investors adopted a cautious stance before a policy update in which the European Central Bank is expected to signal monetary easing ahead while Asian stocks struggled as investors waited for stimulus-sensitive U.S. data.