In the 1990s, a financial advisor analyzed stock and bond returns for every rolling 30-year period from 1926–1955 to the then present. The advisor found that a 60/40 mix (60% S&P 500 and 40% medium-term government bonds) would allow a retiree to take a 4.15% annual withdrawal, adjusted for inflation, for at least 30 years. The unprecedented consecutive three-year decline in the stock market starting in 2000 changed everything.
For example, consider a $100,000 55/45 (stock/bond) portfolio on January 1, 2000, that is rebalanced monthly and includes a 4% annual withdrawal rate plus a 3% increase each year for inflation (e.g., $4,000 the first year, $4,120 the second year, $4,244 the third year, etc.). By 2010, the portfolio’s value would have declined by a third, and there would only be a 29% chance of making it through three decades (source: T. Rowe Price).
Market returns during the initial years of withdrawal can have a huge impact on portfolio sustainability. Three ways to minimize such an impact are as follows:  use an immediate fixed-rate annuity instead of bonds,  use a variable annuity with a guaranteed withdrawal benefit to replace part or all of the portfolio’s stock portion,  use IRS life expectancy tables and portfolio value every December 31 to determine next year’s withdrawal, or  use stock valuations (P/E ratios) to adjust the portfolio’s equity weighting.
The shortcoming of 
above is there may be little, or no, principal remaining with the annuity
upon the owner’s death—this is fine for someone single (or married if a joint benefit option is used) if there is no donative intent. Strategy 
means withdrawal amounts are at the mercy of portfolio returns—a $1 million portfolio and an IRS life expectancy of 23 years translate into $43,478 one year
, but the next year’s portfolio may be worth, say, $900,000 and the life expectancy might be 22.4 years ($900K/22.4 = $40,179 income that year). In a similar vein, strategy 
means increasing the portfolio’s stock portion when the market is considered undervalued (e.g., an S&P 500 P/E of 12) and reducing stock exposure when it is considered overvalued historically (e.g., an S&P P/E of 21).