Warren Buffet is a full time investor, so that's a viable strategy for him. For most people "invest in what you know" is equivalent to "invest in the industry y

Warren Buffet is a full time investor, so that's a viable strategy for him. For most people "invest in what you know" is equivalent to "invest in the industry you work in" which is a bad strategy. Much better is to invest in low-cost index funds, a strategy that could be summed up as "don't know what you're investing in."