Consider an analysis for a rock weighing 500kg, with a standard deviation of 10kg. RocFall performs a Monte-Carlo simulation and samples from that distribution. For every simulation, the distribution is sampled to get a value of the rock mass. In general, 99% of all values in a normally distributed random variable lie between 3 standard deviations on either side of the mean. 67% of all values lie within one standard deviation. So, for this example, 99% of the values will lie within 470kg and 530kg, with the majority (67%) being between 490kg and 510kg.