The University of California plans to double the endowment&rsquo;s investments in private equity and trim stock holdings, the latest move as it tries to improve performance.<br /> <br /> The $9.9 billion endowment had a 7.1 percent investment gain for the six months through Dec. 31, propelled by global stocks and private equity.<br /> <br /> Endowments of all sizes gained a median 4.4 percent in the same time, according to Wilshire Trust Universe Comparison Service, a database provided by Wilshire Associates.