@scottish Historical back testing generally assumes an even distribution of retirement events. But, in practice, people chasing a magic number tend retire at high valuations, rather than low. The article linked by
@FarFetchd explains why. This will be skewed based on how much of a retiree's portfolio growth is based on savings vs. gains, per the post
@shuffler linked.
Retiring during a period of high valuation increases the chance of failure for a given SWR. Sequence of return risks are stacked against you, because the market will most likely revert to mean valuations. Our current valuations are extremely high - there are few comparable historical periods. Today's environment is even more challenging, in that bonds are also at historically low rates. So they don't have much room to appreciate. Oh, and the government is doubling down on MMT due to the pandemic and administration - printing lots of money.
The odds are in favor of us playing near (maybe even beyond) one of the back tested failure scenarios. These are roller coaster experiences to say the least, even if you don't ultimately fail. If you pick a strict % based withdrawal rate, actual withdrawal amounts are going to be highly volatile. They are also likely to drop strongly during your prime health years. If you pick a strict % of original balance, you hit some scary low net worth numbers. These risks are discussed in the article I linked to.
For me personally, smoothing this ride is very appealing. The article I linked to suggests a formula for doing so:
SWR = A + (1/CAPE) * B
So when valuations are high (CAPE) you withdraw a smaller percentage than when they are low. The factors of A and B to use depend on your personal scenario. Age, capital preservation needs, other expected income, etc. ERN provides a spreadsheet for estimating this yourself:
https://earlyretirementnow.com/2018/08/29/google-sheet-updates-swr-series-part-28/What I really like about this, is when the next recession does come, I already have a strategy for how to react. I have a basis to increase my withdrawal rate %, keeping spending closer to my initial retirement levels. I also have selected a more conservative spend relative to my current portfolio value, since I am retiring into a high CAPE environment. Instead of being scared into continuing to work, because of a looming downturn, I can pick a number considering valuations that tests well.
Along a similar train of thought - tax planning strategy also substantially influences the value of the magic number. Let's say you have a million dollars and pick A=1.75, B=0.5. Today's SWR is roughly 3.25%. So $32,500 of annual spend. If you can hit 0% taxes, you get it all. Make other choices (say all your money is in your 401k) and end up paying 15% taxes, well now you can only spend $27,625. Two very different experiences from the same portfolio.
In practice, I think once a retiree gets close to pulling the trigger, they need to do this more careful analysis. Alternatively, they can pick a very conservative spend and over-save. But that is more time working.