How can you possibly model a 60-year investing lifetime (30 years of accumulation and 30 of retirement) using only data since 1970?
You can’t, and the calculator does not try. It takes a different approach.
It’s true that the calculator does not look at full consecutive 60-year accumulation and retirement market sequences, and any single historic lifetime is impossible to accurately represent in the data set. That’s not the goal, however, as the chart displays the full range of accumulation paths using every possible start year to fund the all-time minimum withdrawal rate. The purpose of the calculator is to assume your own sequence of returns will be unknown and to paint the big picture rather than represent any single investing lifetime with precision.
Why do you assume that an investor will have the same portfolio their entire life? Don’t most people recommend getting more conservative as you age?
This deserves its own future post, but let me give a quick summary.
Dr. Wade Pfau has presented good evidence that typical equity glide path advice has very little effect on safe withdrawal rates. So simply following traditional advice pales in comparison to taking a more holistic and diverse approach to asset allocation in retirement.
I also personally believe that developing good habits takes practice, and chasing returns or assuming that your current portfolio is only temporary are counterproductive to that process. Most people would be far better off both emotionally and financially focusing that optimization energy on minimizing expenses and maximizing income from their career. So don’t think of it as investing in a risky way in retirement. Think of it as investing conservatively and sustainably not only in retirement but also in accumulation.
That said, investors wanting to explore multi-allocation strategies are encouraged to use the Portfolio Growth and Withdrawal Rates calculators to study their ideas in depth.
Why do you use the 40-year safe withdrawal rate?
The default setting is the 40-year Safe WR, which is the rate at which the single worst rolling retirement period survived a full 40-years. That seems like a reasonably conservative number for general study, but when in doubt plan conservatively. Using the Portfolio Growth and Withdrawal Rates calculators can help you study different withdrawal rates to find something that gives you confidence in your plan.