Terms

# Modified Dietz Method: Definition and Use in Investing

## What Is the Modified Dietz Method?

The modified Dietz method measures a portfolio’s historical return based on a weighted calculation of its cash flow. It considers the timing of cash flows and assumes a constant rate of return over a specified period.

This method is more accurate than the simple Dietz method, which assumes that all cash flows come from the middle of the evaluation period.

## Understanding the Modified Dietz Method

The modified Dietz method accurately reflects an individual’s personal rate of return from an investment. It considers the market value of holdings at the beginning and end of a period, all cash flows during that period, and the duration of each cash flow event in the account.

### Key Takeaways

• The modified Dietz method is widely used by investment companies to report results to clients.
• It provides a more accurate reflection of an individual’s rate of return.
• The method excludes external factors that can skew the numbers.
• Cash flow can include contributions, withdrawals, or fees.

The number achieved using the modified Dietz method is sometimes called the modified internal rate of return (MIRR), which is a metric used in capital budgeting decisions.

Regardless of its use, the goal of measuring the internal rate of return is to exclude external factors that could affect the results.

### Why was this Method Adopted?

Financial industry watchdogs and investors are increasingly seeking transparency in calculating and reporting investment returns. The modified Dietz method is widely recognized as a step towards improved investment portfolio attribution reporting, and it is commonly used in the investment management industry.

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The result of using the modified Dietz method is sometimes referred to as the modified internal rate of return.

The method is a dollar-weighted analysis of a portfolio’s return, making it a more accurate measure than the geometric return method. However, it can face issues during periods of high volatility or multiple cash flows within a specific period.

This approach to return calculation is similar to the dollar-weighted return method but does not require finding the exact rate of return.

Today, calculating a true time-weighted return is relatively easy by calculating a daily return and geometrically linking them for longer periods. However, the modified Dietz method remains useful due to its performance attribution calculation benefits, which time-weighted methods lack.

This return calculation method is a hallmark of modern portfolio management. It is one of the recommended methodologies for calculating returns by the Investment Performance Council (IPC) as part of the Global Investment Performance Standards (GIPS). These standards aim to provide consistency in international portfolio return calculations.