A private research group said on Thursday that its index on future U.S. economic activity rose in December, suggesting the economy will strengthen over the next few months. The Conference Board said its index of leading economic indicators rose 1 percent last month after a 1.1 percent increase in November, making them the biggest increases since March, when the index jumped 1.4 percent. Economists expected a lower reading of 0.6 percent for December. The measure has stalled in previous months as Europe experienced a debt crisis and a weak job market affected U.S. stocks. But by the end of 2010 the measure rose as U.S. economic conditions improved. Six of the measure's 10 components increased last month, while two declined, and two were steady. The biggest jump was in building permits, suggesting that home construction will rise. The National Association of Realtors said Thursday that home sales rose 12.3 percent in December, the fastest pace in seven months. However, the Conference Board's economists cautioned that the economy still faced big problems in 2011, since state governments are cutting spending, hiring is weak, and home foreclosures are at record highs.