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Safe Withdrawal Rates  Part 4: Perpetual Withdrawal Rates
Recall from Part 1 that the Trinity study defined a withdrawal rate as “safe” so long as the portfolio always had enough to make withdrawals during retirement. That means if the portfolio was worth exactly $0 at the end of the retirement duration, that would count as a success. We’ve been using that same definition in the other parts of this series.
However, many retirees would not consider such an outcome a “success”. Watching your balance drop down closer and closer to 0 would be quite nerveracking. Moreover, many people want to be able to leave assets to their heirs or charities upon their death.
There’s an easy (albeit conservative) way to adjust Trinity style safe withdrawal rates to guarantee a “minimum” floor for assets at death. Suppose the retiree has X dollars, and wishes to ensure that Y inflationadjusted dollars will be available for heirs. They can simply put the Y dollars in an inflationhedged instrument like TIPS or IBonds, which give a guaranteed real rate of return over inflation. Then, they can apply the usual SWR methodology to the remaining (X  Y) dollars.
However, setting an arbitrary “base” floor that assets should never dip below seems slightly arbitrary. What if we want to ensure that the whole principal remains in tact? In other words, what if we want a perpetual withdrawal rate that can last forever.
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Safe Withdrawal Rates  Part 3: More Bootstrapping
As we saw in the previous post, the “standard” approach of choosing a safe withdrawal rate based on what has never failed in the past is questionable. The reason is that we simply don’t have that much data. If you had tried this approach in the past, by looking at what had never failed as of 1980, you would have overestimated the actual SWR and encountered a failure.
To try to address this, we used the stationary bootstrap to simulate alternate possible returns data. For each simulated data set, we reran the Trinity study methodology in order to see how different the “no failure” rate would have been. We found that in a nontrivial percentage of simulations, the “no failure” rate was much, much lower than the standard recommendations. This suggests that there is a fair degree of variability in what the Trinity study methodology could have reasonably found. So, just picking something that has never failed before does not give a good sense of how secure that withdrawal rate really is.
To do better, we’ll again turn to the stationary bootstrap. This time, we’ll use it to estimate safe withdrawal rates directly, rather than to study the statistical behavior of the Trinity study methodology.
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Safe Withdrawal Rates  Part 2: Do we have enough data?
Last time, we reviewed the definition of safe withdrawal rates and explained the origins of the popular 4% rule. The idea was to estimate how much we could safely withdraw postretirement by simulating how retirees from 1926 through 2015 would do. For each possible year in that range and possible retirement length, we checked whether a retiree who started then and withdrew a given amount each month would run out of money. As expected, the longer the desired duration of the retirement, the less we could safely withdraw each year. This is especially important for people who want to retire early, because many studies of safe withdrawal rates focus on “normal” retirement lengths of at most 30 years.
When we consider longer retirement lengths, a serious question is whether we have enough data to make a safe conclusion. For example, someone retiring early at age 35 who wants to ensure they have enough money till 95 is interested in a 60 year safe withdrawal rate! But if we’re using stock data from 1927 to 2015, we have less than 90 years of data. Moreover, if we follow the Trinity study methodology of examining the outcome of each possible starting year, the last cohort we can consider is 1955 – for every starting year after that, we need data beyond 2015 to know whether they would have made it. (Of course, if they’ve already run out of money, we can know that would have been a failure. But we can’t tell how the remaining cases will turn out. I do not count such failures because it appears that the Trinity study did not either.)
So what can we do? One possibility is to try to extend the data. I’m writing this in 2019, so we could update our ending date data by a few more years. From the other direction, some people have figured out estimates of stock and government bond returns from all the way back to 1871. That’s a lot of additional data! (Although one might wonder how relevant such old estimates are for figuring out how the market behaves today.)
While trying to get more data is a good idea, in this post I want to look into another matter: can we estimate how much a lack of data may be hurting us? And, can we extract even more insight from the data we do have?
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Safe Withdrawal Rates  Part 1: The Basics
The study of safe withdrawal rates helps answer two crucial questions about retirement savings and planning:

How much money do we need to save for retirement?

How do we make sure that our savings last throughout our retirement?
Of course, these two questions are related. If we save too little, then it will be a struggle to stretch our savings out during retirement. On the other hand, mismanagement and excessive early spending could deplete even a seemingly large nest egg. Proper planning will help us figure out what savings rate we need to hit our targets and know when we are financially ready to retire.
To get started answering these questions, we first need to have some estimate of what our expenses will be after retirement. This itself can be tough to figure out. In the end, this is highly personal and will vary for each individual. Moreover, expenses might change throughout retirement, sometimes dramatically so. In order to make any progress, we’ll have to make some simplifying assumptions.
You may have heard of the 4% rule, which says that you can safely take out 4% of your original savings every year, with adjustments for inflation. For example, if you retire in 2020 with $1 million dollars in your portfolio, then the first year you withdraw $40,000. Every year after, you convert $40,000 in 2020 dollars into the current year to adjust for inflation and then withdraw the adjusted amount.
But is this really safe? Well, it depends. The rule relies on certain assumptions. Unfortunately there is lot of misunderstanding about this on reddit and financial independence blogs! To clear this confusion up, let me explain a little bit more about where this rule comes from.
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Review: A Random Walk Down Wall Street
A Random Walk Down Wall Street, by Burton Malkiel, is a classic book on personal investing. The first edition was published in 1973, and there have since been twelve editions, with the most recent released in 2019. Considering how much the market has changed in that time, its continued relevance is a testament to the book’s quality and central message.
The book’s title is a reference to the Random Walk Hypothesis (RWH), the theory that prices of bonds and stocks can be viewed as random walks, a mathematical model in which prices increase or decrease randomly without regard to their historical value. If this model is accurate, then it’s impossible to predict whether a given stock’s price is about to rise or fall.
In light of that, the book argues that the most sensible strategy is to buyandhold. Over time, the value of one’s investment will grow as prices drift upward. Since any individual stock is still likely to behave somewhat erratically, it is prudent to diversify and purchase stocks from a wide range of companies. The simplest way to do so is to purchase shares of index funds, which buy stocks from all of the companies that belong to an index like the S&P 500. Many of these funds charge very low overheads and in exchange, they handle purchasing all the stocks at the appropriate weights, rebalancing as prices change, and distributing the dividends issued by each stock.
By now, this advice is widespread and popular, but it wasn’t always. When the first edition of A Random Walk Down Wall Street came out in 1973, there was no widely available index fund that one could invest in. The Vanguard Group, which is now one of the largest index fund managers, was founded the year after in 1974. Today, it manages over $5 trillion in assets, and many other large companies also provide funds tracking a wide range of indices.
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