"Vanguard's research found the average five-year annualized return was 21%, but there was wide variation: The five-year annualized return was negative 0.04% at the fifth percentile and a positive 44% at the 95th percentile. 'If you happened to work for a company with returns in the top 5%, you earned a 44% return -- that's pretty sweet' " says Jean Young, senior research analyst in Vanguard's Center for Retirement Research. 'People who see that kind of return become very attached to their company stock, but it's a type of error in portfolio construction. They don't understand the risks they are taking.' "