Before revision:  While randomly selecting a subset of securities from the index increases the chance of outperforming the index, it also increases the chance of underperforming the index , with the frequency of underperformance being larger than the frequency of overperformance .
After revision:  Randomly selecting a subset of securities from the index increases the chance of outperforming the index, it also increases the chance of underperforming the index , with the frequency of underperformance being larger than the frequency of overperformance .