Pop-quiz You worked very hard and earned $1000 that you want to save for your retirement. You are going to retire in 20 to 40 years. What should you do? Keep the money in your mattress. Buy gold. Buy US Treasuries. Buy ETF tracking S&P500. You can jump to the answer right away or keep reading to see the analysis. Problem Setup and Data I would like to investigate what is the probability of a negative return on an investment over a fixed time horizon - holding time.