The rate of annual withdrawals from personal savings and investments helps determine how long those assets will last and whether the assets may be able to generate a sustainable stream of income over the course of retirement. The individuals age and health are important considerations in deciding the rate of annual withdrawals. Assuming average life expectancy may not be advisable in light of probabilities for a healthy adult to outlive his or her indicated average life span. In addition, it may be misleading to expect that investment returns and rates of inflation will tend to mirror their historical averages. A pronounced rise in inflation or negative investment returns due to a market downturn can have a significant impact on the ability of a retirement portfolio to generate income in future years.