U.S. stock index futures turned negative on Friday on doubts about the strength of the economic recovery, after data showed new orders for long-lasting goods unexpectedly fell in August.

S&P 500 futures fell 3.7 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 21 points and Nasdaq 100 futures lost 6.5 points.

(Editing by Padraic Cassidy)