# A Guide to a Conservative Return Assumption

A
simple approach for building a financial plan is to decide on a rate of return
for the investment portfolio and to plug that value into a spreadsheet to
represent assumed asset growth. Historical data may be used to calculate
historical average returns for different asset classes, which are then combined
to create the overall portfolio return. This approach is also known as
deterministic modeling, as there is no randomness in the future outcome. The
same return is obtained each year without variability.

Deterministic
approaches are overly simplified because they do not account for volatility and
therefore miss the impact of sequence-of-returns risk. The basic approach of
assuming a fixed return reflecting the best guess about future market returns
leads to a retirement plan with only a 50 percent chance to work. The outcomes
are too optimistic and could lead a retiree down an unsustainable path.

Monte
Carlo simulations provide an alternative that is now widely used in financial
planning software. Simulations are used to develop sequences of random market
returns fitting predetermined characteristics, in order to test how financial
plans will perform in a wider variety of good and bad market environments. The
use of Monte Carlo tools has increased considerably over the past decade, which
can likely be attributed to lower computing costs, increased recognition that
returns are random, and desires to provide more robust financial plans. A
thousand or more simulations could be created to test the robustness of a
retirement plan in many market environments.

Monte
Carlo simulations can be created for different asset classes or for an overall
portfolio. With the asset class approach, one defines the arithmetic average
return, the standard deviation for that return, and the correlations with other
asset classes. Random draws are then taken from statistical distributions
sharing these characteristics. By combining the arithmetic mean with
volatility, the resulting simulated returns will display the appropriate
compounded return over time. Historical data is commonly used to set these
input characteristics. Most financial planning software works in this way.

With
Monte Carlo based financial planning software, retirees generally focus on
building a plan that achieves a high probability of success, such as 80 or 90
percent. This implicitly means the underlying assumed return is below average. But when thinking in terms of a fixed return
assumption, we usually consider what we view as the best guess for future
returns. Again, the best guess only implies a 50 percent chance for success.
Half of the time, the realized return will be higher and half the time it will
be less. In order to have a conservative fixed return assumption, we must
further scale down from our best guess estimate. This is a point which many
investment management professionals have not internalized into their thinking,
as they are conditioned to using their idea about average returns as the input.

Implied
fixed investment returns are usually not shown with Monte Carlo simulation
output in financial planning software, but they do exist underneath the hood.
We can reverse engineer their values. So which implied portfolio fixed return
supports a 90 percent chance for success? The implied return will be lower than
the average return input for the simulation, and I find support for appropriate
portfolio return assumptions in the postretirement period to be more
conservative than in the preretirement period.

This is an excerpt from Wade Pfau’s book, Safety-First Retirement Planning: An Integrated Approach for a Worry-Free Retirement. (The Retirement Researcher’s Guide Series), available now on Amazon.

Originally posted at https://retirementresearcher.com/a-guide-to-a-conservative-return-assumption/