Coming after a 17-year bull market and just before a decade of stagflation, a December 1968 retirement was the very worst period, no matter what kind of withdrawals you were taking, Wood says. Bengen’s own research has confirmed as much.

Still, if you took out monthly withdrawals or quarterly withdrawals, “it increased the rate you could take out by about 20 basis points,” Wood says. More often than not you were 20 basis points better in the rolling periods, though there were a few instances, he says, when the annual withdrawals would have been better (generally during years of market pullbacks). More often than not, though, you were 20 basis points better off with more frequent withdrawals.

The firm also used different allocations, using a portfolio melding the Russell 2000, the S&P 500 and the Bloomberg Barclays U.S. Aggregate Bond Index, mixing a 50% allocation to the S&P with a 10% allocation to the Russell index and a 40% allocation to the bond index. These allocations were then tweaked (the bond portfolio went anywhere from 30% to 50%).

The math was fairly linear and done in Excel with rebalancing, Wood says.

These findings also have ramifications for the longevity of portfolios, Wood says. In other words, it means something tangible and important for human beings who are living longer. “My great-grandmother recently passed away; she was 97,” he says. “When she was born, her life expectancy was around 60. So it would have been really easy not to protect for longevity for her and I’m glad her advisor did.”

So with the 50% large-cap/10% small-cap/40% bond portfolio, chosen for the worst 30-year period starting December 1968, Wood says 4% annual withdrawals run out somewhere around year 33, plus 10 or 11 months (he adds that other models often don’t include those small-cap allocations). With monthly withdrawals and small-caps included in his model, it’s running out around year 40. “So it’s seven extra years.”

Then you can take the 30-year period starting in September 1, 1929. In that case, the retiree never ran out of money, even after 50 years (the limit for the model). The reasons for that are complicated—one is the performance of small-cap stocks over that period, and the other is the period of deflation it covered during the Great Depression. Annual and monthly withdrawals both survived the turmoil.

Wood says that many of the models for 30-year-return results, more than half of the ones he’s seen, are using just the S&P 500 and bonds. “But our clients are much more diversified than that.” Typically adding other asset classes improves the safe withdrawal rate, he says.

These kinds of tweaks can offer dramatic changes, especially when retiring in one off year can throw off your sequence of returns and determine whether your portfolio is a smooth train ride or a mine car going off a cliff.

Wood takes three years close together as an example. Starting December 1, 1966, with annual withdrawals, the safe withdrawal rate was 4.88%, That number was 4.41% if you retired in December 1, 1967—a 47 basis point difference just one year later—and in December 1968, that safe rate had fallen to 4.09%.