LONDON — The world's wealthiest families plan to allocate more money to private equity after the asset class helped drive an average 15.5 percent rise in the value of their investments in 2017, according to a survey. Some 311 family offices - set up to manage the wealth of one or more rich families - took part in the survey, conducted by Swiss bank UBS, whose wealth management arm manages around $2.5 trillion in assets, and Campden Wealth.