Consistent Inflation Treatment in Long-Term Wealth Simulations

Summary

The document raises a modeling question about projecting wealth over a long horizon with portfolio returns and scheduled cash inflows or outflows. It contrasts reducing nominal investment returns by an assumed inflation rate to obtain real returns with adjusting individual cash flows according to how their values may change over time. Examples distinguish inflation-sensitive living expenses from fixed-interest payments, and include salary and a future donation as planned cash flows.

The central modeling concern is whether inflation should be applied to returns, cash flows, or both. The document itself does not include an answer, calculations, or evidence resolving that choice, so it functions as a framing of the consistency problem rather than a validated method. A useful analysis would need to specify whether wealth is measured in nominal or purchasing-power terms, and how each flow and return is denominated. Its proposed geometric Brownian motion setup is mentioned but not examined for suitability or parameterization.

Key ideas

• A wealth projection can represent portfolio returns and external cash flows separately.
• Inflation may affect investment returns and different cash flows in different ways.
• The model must keep nominal and real units consistent throughout the projection.
• The document poses the issue but supplies no answer or empirical assessment of its proposed approach.

Tags

# Inflation in wealth forecast


# Inflation in wealth forecast












I am building a model to simulate people's wealth in the next years.

Say Mr X has a portfolio with an expected return of 3% (annual). From this I can simulate the return of his portfolio in the next 40 years using Geometric Brownian Motion. I also want him to be able to add inflows/outflows (e.g. Mr X expects a salary of 60K every month and expects to receive 100K as a donation in 5 years).

I now want to adjust all this with inflation.

In my view there are 2 ways to do so:

- I can adjust the portfolio return. So if I consider 2% inflation per year, the real return rate would be 1%.

- I can adjust the inflows/outflows. This approach in my view is a bit more precise, since some cash flows may be impacted by inflation (such as expenses for life goods) and others not (such as fixed interests).

My question: I believe I should adjust inflation doing either 1) or 2) but not both ways simultaneously. Do you agree with this?

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This summary was written by Stratmill's research agent from the original; it is not a copy of the source.