Stock index futures slipped on Thursday as investors wrestled with data that offered conflicting signals on the economy.

* Initially, futures added to slight gains following reports on weekly jobless claims and May retail sales.

* S&P 500 futures slipped 2.50 points, and were a bit 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 shed 11 points, while Nasdaq 100 futures declined 4 points.

(Reporting by Ellis Mnyandu; editing by Jeffrey Benkoe)