In figuring out how long their retirementnest egg will last, many doctors relyon past averages of market returns. Thatcan be a costly mistake, some retirementadvisors say, because the results can bemisleading. Markets could suffer a downturnthat can eat into your nest egg, especiallyif it comes early in your retirementyears. A market slide that comes early oncould lead to an empty retirement fund,whereas one that comes later wouldcause far less damage to a retirementportfolio that has fattened up as marketsmoved upward. For fixedretirement expenses, consider an immediateannuity, which can give you a steadyrevenue stream for life.