In the long run, the Social Security Administration assumes the “mortality improvement rate” will be 0.71 percent — that is, the odds of dying at a given age will fall, on average, that much each year. Remember, that’s a rate of relative improvement, and at most ages the odds of dying are low, so the numbers won’t move around very much. If a person your age had a 1 percent chance of dying in a given year, the S.S.A.’s assumption of 0.71 percent annual mortality improvement would imply a 0.9921 percent chance of death the following year for people a year younger than you — just a little bit safer.