Considering 30-days investment horizon, iShares Core S P MidCap ETF is expected to under-perform the SP 500. In addition to that, iShares is 1.1 times more volatile than S&P 500. It trades about -0.16 of its total potential returns per unit of risk. S&P 500 is currently generating about -0.12 per unit of volatility. If you would invest  165,535  in S&P 500 on May 21, 2013 and sell it today you would lose (2,642) from holding S&P 500 or give up 1.6% of portfolio value over 30 days.