Risk Manager
A view from the trenches on the benefits and drawbacks of Monte Carlo simulation
By Mike Patton
View Mike's most recent musings in his blog
From the September 2006 Issue of Investment Advisor Magazine
When financial planners talk about how the profession has advanced over the years, one of the prime examples given is that of Monte Carlo simulation (MCS), which over the past 15 years has found its way into many financial planning software packages.
The software has its origins in the 1940s at the Los Alamos National laboratory in New Mexico, where scientists created a computer program to model the range of possible outcomes of a nuclear explosion. They named the program Monte Carlo after the quarter of Monte Carlo, in the Principality of Monaco, where many try their luck at the famous roulette wheels (which, of course, are based on chance).
These days, the majority of planning programs incorporate MCS to some degree, but this method of calculating probability may still be in the earlier stage of developmental use. Like the American pioneers of old, there are several new frontiers to explore and questions to address before planners understand and apply MCS appropriately. These would include determining how many trials should be run and which variables to include in simulations, and whether some planning software might be yielding an incomplete picture by applying MCS only to investment returns. It’s also important to explore the probability of the linear forecast and determine why such a forecast is important. This article will attempt to address these issues in a practical manner, using examples that we all might see with our clients.
The benefits of Monte Carlo
There are several different ways to “project” future outcomes. These methods include linear forecasting, time series forecasting, Latin hypercube, time path analysis (looping or rolling), and Monte Carlo simulation (MCS). In terms of MCS, three frequently used forecasting methods are parametric, non-parametric, and economic modeling. We will focus on the parametric method of forecasting.
Risk is a primary reason why we use MCS. Risk can either be subjective or objective, significant or insignificant. Some risks are objective, such as flipping a coin. It doesn’t matter if the first 10 flips were heads because the next flip of the coin still has a 50/50 chance of being either heads or tails. An example of subjective risk would be predicting the weather. Given the same data, two different weathermen may forecast different chances for rain. A significant risk would be a tightrope walker performing 500 feet above the ground without a net. An insignificant risk would be the same tightrope walker traveling only one foot above the ground.
Risk stems from our inability to see into the future. Newton’s third law of motion states that “for every action there is an equal and opposite reaction.” There may be a modified application here for Newton’s law outside the realm of physics. Here are some examples: If I increase my investment risk, I expect to increase my return by some factor; If I don’t buy that new car today, but instead invest the money, I should have more money tomorrow. These are just a few of the many decisions that we and our clients face. MCS can be of great help in making these decisions.
Average Isn’t Good Enough
Many of today’s software programs tend to use MCS primarily around investment returns. Additionally, they may categorize a particular holding as a large-cap stock, small-cap stock, intermediate term bond, etc., and impose the standard deviation (risk) for the entire category on that particular holding. While it’s prudent to develop sound assumptions around risk and return, this approach can be problematic. For instance, the stock of a company with a $50 billion market cap would be considered a large-cap stock by most observers. Let’s say the average standard deviation for large-cap stocks was 20%. What if that particular stock’s standard deviation was 40%, 50%, 60%, or higher? In such a case, we would be greatly understating the risk. Here’s the irony. The reason our industry has gravitated to MCS in the first place is because forecasting using linear assumptions can be misleading! Most planners would recognize the problem in relying on averages when forecasting, since the forecasted results rarely materialize as expected. Yet some continue to use averages by categorizing an asset and using the category average as a proxy for a particular holding.
To further illustrate this problem, consider the “Averages” table above, right. According to Morningstar, as of Dec. 31, 2005, there were 6,423 domestic stocks. This number included small, medium, and large companies. The average standard deviation for this universe was 27.40%, with a high of 964.9% and a low of 6.60%.
