An investor is analyzing the risk of a possible investment by producing three different scenarios. Under a pessimistic scenario, the property would produce an ATIRR of 8 percent; a most-likely scenario would produce an ATIRR of 12 percent; and an optimistic scenario would produce an ATIRR of 16 percent. The investor assigns the pessimistic scenario a 25 percent chance of occurring, the most-likely case a 60 percent chance of occurring, and the optimistic scenario a 15 percent chance of occurring. What is the standard deviation of the returns?