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cFIRE question about monte carlo versus historicals
Old 01-09-2015, 12:26 PM   #1
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cFIRE question about monte carlo versus historicals

I ran cFIRE with about $3MM in assets, SS for me and the spouse out a ways, $130k/yr spending, 36 year span, 50/50 stocks/bonds. I chose to run it using Historical Data - All Years. It came back at a 92.6% success rate.

Then I ran it using Monte Carlo. I just used the default values (6.4% equity gain/1% st dev, 1.8% bond gains/5.1% st dev). The success rate dropped all the way down to 40%.

I'm not smart enough to know what that means and if that means my plan is a bust. Can anybody help?
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Old 01-09-2015, 12:28 PM   #2
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monte carlo simulation probably doesn't have mean reversion programmed - you can get some wacky results with monte carlos - even just going 10 years out
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Old 01-09-2015, 02:18 PM   #3
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Be sure to run any Monte Carlo sims several times as you can get a range of results.


If using MC, get an idea of the range of % success results - you can get a false sense of security if, by chance, you run it once and it shows 100% success, cuz the next time you run it, the results may be shockingly low.
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Old 01-09-2015, 02:22 PM   #4
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It's back to 97% success under MC. I had some large "other incomes" coming in one year later later in the simulation that I think screwed it up. I simplified it to just one portfolio in year 1 and MC always comes back over 90%
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Old 01-09-2015, 02:57 PM   #5
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I'd use Flexible Retirement Planner for MC simulations. It will run 2000 (I think) and give you the 10/90% bands as well as the number of times money actually ran out in those simulations and the %age in each year.
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Old 01-10-2015, 04:54 AM   #6
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I ran cFIRE with about $3MM in assets, SS for me and the spouse out a ways, $130k/yr spending, 36 year span, 50/50 stocks/bonds. I chose to run it using Historical Data - All Years. It came back at a 92.6% success rate.

Then I ran it using Monte Carlo. I just used the default values (6.4% equity gain/1% st dev, 1.8% bond gains/5.1% st dev). The success rate dropped all the way down to 40%.

I'm not smart enough to know what that means and if that means my plan is a bust. Can anybody help?
the problem with historical is alot of it depends on time frames playing out in the same order.

each 30 year period is really dependent on exactly how the time frames before it and intersecting with it behaved.

as an example 1987 to 2003 saw the greates bull market ever . 17 years of almost 14% returns.

as pfau said "

Between 1926 and 2010, there are only 56 rolling 30-year periods. And as is about to be discussed, these 56 periods are not independent of one another. Meanwhile, it is not uncommon to see a Monte Carlo simulation study based on 10,000 simulated paths of financial market returns"


"because of the way that overlapping periods are formed with historical simulations, the middle part of the historical record plays an overly important role in the analysis. With data since 1926 and for 30-year retirement durations, 1926 appears in one rolling historical simulation, while 1927 appears in two (for the 1926 and 1927 retirees). This pattern continues until 1955, which appears in 30 simulations (the last year for the 1926 retiree through the first year for the 1955 retiree). The years 1955 through 1981 all appear in 30 simulated retirements"


wade pfau looked into this in one of his studies


http://wpfau.blogspot.com/2012/05/mo...istorical.html
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Old 01-10-2015, 09:59 AM   #7
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the problem with historical is alot of it depends on time frames playing out in the same order.

each 30 year period is really dependent on exactly how the time frames before it and intersecting with it behaved.

as an example 1987 to 2003 saw the greates bull market ever . 17 years of almost 14% returns.

as pfau said "

Between 1926 and 2010, there are only 56 rolling 30-year periods. And as is about to be discussed, these 56 periods are not independent of one another. ...
IMO, this is the value of historical analysis, not a problem at all. I don't think market cycles are independent, that's why they are called cycles, so why use a model that treats them as independent?

We get bubbles, bubbles burst. Bad periods followed by recovery. Recovery followed by a slow down. Of course, a 'near-end-of-the-world' type scenario could occur, and there would be no recovery (in our lifetimes at least), but you can only plan for so much before you decide to work until you die.

I think MonteCarlo is the problem - it does not account for these cycles, or does so artificially in some way often only known to the programmer (and maybe not even to the programmer).

An ironic justification of MonteCarlo is when it is mentioned that the results are in-line with historical analysis! Then why not just use the historical analysis?

-ERD50
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Old 01-10-2015, 10:28 AM   #8
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...

An ironic justification of MonteCarlo is when it is mentioned that the results are in-line with historical analysis! Then why not just use the historical analysis?

-ERD50
As I understand it, and someone correct me if I'm wrong, the reason for using MC with historical is to account for sequence of returns. Pfau, Cotton, or Kitces (or someone) talks about this somewhere but I'm too lazy to search for it . I think it was Pfau that talks about fat tails accounted for in MC simulations, but I'm not sure. Dirk Cotton addresses strategies for overcoming SOR risk here:

The Retirement Cafe: RIIA Webinar -- Sequence of Returns Risk

To all of this, I personally would never be comfortable with a mere 100% success rate in any calculator. All calculators I use provide for generous heaping of $$ left over at end of plan, aka, "padding" (in addition to the padding in my budget).

Remember, we are engineering for the absolute worst case scenario. RIP provides results accounting or average, below average, and signficantly below average (which I use), as does Financial Engines, which I use free through VG. ESPlanner, if you use the most conservative mode, can provide results for earning returns just keeping with inflation in Monte Carlo, or complete loss of risk assets in upside planning (I model both of these modes as they are the most conservative in ESP). Firecalc uses historical, while you can fiddle with cfiresim to produce all kinds of juicy results.

In all probability, if we model conservatively, we will probably have more money than we know what to do with in later years. Visited the folks over the holidays and this is precisely the "predicament" they are in.
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Old 01-10-2015, 10:03 PM   #9
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I find it intriguing how revered these probablaistic calculators seem to be. These are mere models of what we might expect. They are flawed from the outset in several important way:


1) They are based on historical data (either directly in historical methods or through estimating the paramets of a MC simulation)

2) That they don't model rational retiree behaviour. I don't think any reasonable person will continue to spend themselves into poverty as is depicted in these models.

These tools are rough measuring instruments. The give you some confidence that you are in the ballpark. Seeking 100% success in the model and lamenting anything less is a bit over the top IMHO. I rather like the idea of insuring your basic needs through pension/SS. etc as much as possible as the "padding" in the retirement plan and following a variable spending approach..
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Old 01-11-2015, 02:47 AM   #10
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you do not want to introduce the variables of spending patterns into the equation. then you cannot compare or identify mathamatically what any of the results mean.

it is spending patterns that can make things better while the numbers themselves are based ON WORST CASE SCENARIOS.

that is kind of like having a built in buffer for those big unexpected expenses.

the fact we do not generally need anywhere near all that inflation adusting every year as we age on top of human spending patterns that have us cuting back by comfort level when markets are down help imorove success rates.


if anything if we have average markets and inflation it will be a pleasent upside surprice instead of a big disapointment if we don't and possibly catastrophic consequences.
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Old 01-11-2015, 12:34 PM   #11
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you do not want to introduce the variables of spending patterns into the equation.

...
Yes you can, and many do:

Jonathan Guyton on Combining Dynamic Withdrawals and Dynamic Asset Allocation

cFIREsim just happens to provide an ability to model this type of spending approach, whereas FIRECAL does not.
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Old 01-11-2015, 05:31 PM   #12
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the traditional studies did not take human spending into consideration.
many planners today use lifestyle planning . after taking the results from studies like the trinity software adjusts thing to fit your lifestyle.

want more early on for travel and less later? the sofware can give you a road map.
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Old 01-11-2015, 07:12 PM   #13
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the traditional studies did not take human spending into consideration.

...
This is known. In fact the author of either the original Trinity study or Bengan's follow up study (forget which) later advised that one should be flexible and not take inflation adjusted withdrawals blindly. And yes, CFIREsim can model this and other spending approaches (I particularly like this feature). I agree with your excellent way (IMO) of viewing spending in your above post:

Quote:
it is spending patterns that can make things better while the numbers themselves are based ON WORST CASE SCENARIOS.

that is kind of like having a built in buffer for those big unexpected expenses.

the fact we do not generally need anywhere near all that inflation adusting every year as we age on top of human spending patterns that have us cuting back by comfort level when markets are down help imorove success rates.


if anything if we have average markets and inflation it will be a pleasent upside surprice instead of a big disapointment if we don't and possibly catastrophic consequences.
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Old 01-11-2015, 08:59 PM   #14
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Where can I find cFIRE?
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Old 01-11-2015, 09:46 PM   #15
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Where can I find cFIRE?
Google would find it.

Here it is...
Crowdsourced Financial Independence and Early Retirement Simulator/Calculator
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Old 01-11-2015, 09:46 PM   #16
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Where can I find cFIRE?
There is a link at the bottom of this page to FIREcalc
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Old 01-12-2015, 09:59 AM   #17
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I had done a Google search but did not see it so thanks for the link.

I have used FIRECalc in the past, but cFIRE appears to be different.
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Old 01-12-2015, 10:04 AM   #18
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I have used FIRECalc in the past, but cFIRE appears to be different.
You might find this interesting: cFIRE vs FIRECalc?
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Old 01-12-2015, 02:54 PM   #19
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Thanks for the post, REW. I hadn't used CFIRE in a while, and when I went back to it, noticed it had improved a great deal. I believe the improvements in modelling were added after the 3/14 thread as I posted there and would have mentioned the improvements (which I really like), but did not at the time.
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Old 01-15-2015, 04:46 AM   #20
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I had done a Google search but did not see it so thanks for the link.

I have used FIRECalc in the past, but cFIRE appears to be different.
Sorry Raymond , Crowdsourced Financial Independence and Early Retirement Simulator/Calculator , hope this is what you were looking for.
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