Stock index futures pointed to a flat open on Tuesday ahead of key data on productivity and labor costs, and before the Federal Reserve kicks off a policy meeting.

S&P 500 index futures were down 2.3 points, and above 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 11 points, while Nasdaq 100 futures fell 0.25 points.

More evidence of an economic recovery could come from productivity and unit labor costs data due at 8:30 a.m. EDT. Economists in a Reuters survey forecast a productivity rise of 5.3 percent versus 1.6 percent increase in the first quarter. Unit labor costs are seen down 2.4 percent compared with a 3.0 percent rise in the previous quarter.

The U.S. Federal Reserve kicks off its two-day monetary policy meeting. While interest rates are likely to remain at current levels, the focus will be on signs of an exit strategy from the Fed's quantitative easing policy.

Other economic data will be chain store sales at 7:45 a.m. EDTfrom ICSC/Goldman Sachs for the week ended August 8. In the previous week, sales fell 0.2 percent.

Redbook releases its retail sales index of department and chain store sales for August versus July. The data is due at 8:55 a.m.

The Commerce Department reports wholesale inventories for June at 10:00 a.m. EDT. Economists in a Reuters survey forecast inventories to fall by 0.9 percent in June versus a decline of 0.8 percent in May.

Investors will get a better idea of consumer spending when ABC News releases its consumer comfort index for the week ended August 9. In the previous week, the index read -49. The data is due at 5:00 p.m.

On the earnings front, Applied Materials Inc , the world's largest chip equipment maker, reports third-quarter earnings after the bell.

Stocks pointing to a lower open included Rino International Corp , off 18.2 percent after the closing bell on Monday after the company reported results.

Seattle Genetics shed 8.8 percent after news of a proposed public offering of common stock after the close.

Quest Software Inc rose 5.8 percent to $15.80 after the close after it posted a better-than-expected second-quarter profit.

Wall Street ended lower Monday after investors sold off some equities after a four-week rally took the broad S&P 500 index to a 10-month high last week. The index is now up 48.9 percent from its 12-year closing low set on March 9.

(Reporting by Angela Moon; editing by Jeffrey Benkoe)