I teach cost-benefit analysis, which requires me to teach monte carlo simulation for sensitivity analysis. I use excel. I understand how to generate a number with uniform, triangular, normal or other distributions, but I don't know how to randomly generate a set of numbers between zero and one which sum up to one. Here is the exact application. Suppose I have an estimate of the proportions of blacks, whites, hispanics, and asians in a given population, let's say 0.2, 0.5, 0.2, 0.1. In my cost-benefit analysis there is some impact I'm estimating that depends on these proportions. For example, suppose I'm estimating the total number of people who will sign up for medicaid benefits, and I have an estimate of the sign-up rate for each race/ethnic group, so the total number of sign-ups depends on the weighted average of sign-up rates. In monte carlo sensitivity analysis I want to vary each of these parameters over some believable range. Let's say I have reason to believe that the race/ethnic proportions may not be exactly 0.2, 0.5, 0.2, 0.1, but that each of them lies in a range of .05 above or below those numbers, so my ranges are [0.15,0.25], [0.45,0.55], [0.15,0.25], [0.05,0.15]. If I naively tell excel to choose four numbers randomly, one from within each of those ranges, they are extremely unlikely to sum up to one. If I tell excel to choose a number from each of the first three ranges and subtract their sum from one to get the fourth number, it is possible the first three will sum to more than 1. Ultimately what I need is to be able to randomly generate a set of proportions that sum up to one when I have some belief about the range each proportion must lie in. I have no idea how to think about how to do this, but it must come up a lot. I (and my students) will be hugely grateful for a solution. Dan Acland, Goldman School of Public Policy, UC Berkeley.