BOULDER — Strong stock performances in 2012 by about half of the publicly traded companies based in the Boulder Valley helped the group outperform the three major U.S. stock indexes.

Six of the 11 companies that call either Boulder or Broomfield counties home ended the year with improved stock prices: Ball Corp., Level 3 Communications Inc. and Vail Resorts Inc. in Broomfield, Clovis Oncology Inc. and Array BioPharma Inc. in Boulder, and Longmont-based DigitalGlobe Inc.

The combined values of stocks of public companies based in the Boulder and Broomfield counties increased 16.2 percent from Jan. 3 to Dec. 31, 2012. The increase is a sharp contrast to the 17.7 percent decline the group had in 2011. It was more in line with 2010 when the combined value increased 29.9 percent, and 2009 when it increased 17.4 percent.

This percentage increase does not reflect an index, but provides a reasonable snapshot of the performance of stocks of these local companies.

For the year, the Dow Jones industrial average increased 7 percent, Standard & Poor's 500 index gained 13 percent, and the Nasdaq composite index was up nearly 16 percent.

The area lost one publically traded company, but it gained another. Boulder-based New Frontier Media Inc. was acquired in October by California-based LFP Broadcasting LLC . Broomfield-based WhiteWave Foods Co. became a publicly traded company Oct. 26.