What is the Modified Dietz method and when is it used?

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AcadiFi · Accepted Answer

Modified Dietz is an approximation to time-weighted return that doesn't require daily valuations. It's widely used when sub-period valuations at each cash flow date are unavailable. Before daily valuation became standard, Modified Dietz was the industry workhorse; today it remains common for periods with infrequent valuations.

Formula: R = (MV_end − MV_begin − CF_total) / (MV_begin + Σ wᵢ × CFᵢ)

Where wᵢ is the weight of cash flow CFᵢ representing the fraction of the period the cash flow is present: wᵢ = (CD − Dᵢ) / CD, with CD the total days in period and Dᵢ the day of the cash flow.

The numerator captures total gain. The denominator captures average capital employed, weighting each cash flow by the time it was in the portfolio.

Worked example: Linnea Advisors reports an account for Q2.

- Apr 1 (day 0): MV = $500,000
- May 15 (day 44): $50,000 contribution
- Jun 30 (day 91): MV = $565,000

Cash flow weight: w = (91 − 44) / 91 = 0.5165

R = (565,000 − 500,000 − 50,000) / (500,000 + 0.5165 × 50,000) = 15,000 / 525,825 = 2.852%

Accuracy: Modified Dietz is accurate when: (1) cash flows are small relative to portfolio, (2) return is relatively stable within the period, (3) cash flow timing is roughly uniform. It loses accuracy with large cash flows during volatile periods — exactly when true TWR matters most.

Original Dietz (not modified) assumes all cash flows occur mid-period, giving weight = 0.5 for every cash flow. This simplification is cruder but was standard before the 1980s.

GIPS compliance: through 2001, Modified Dietz was acceptable. From 2005, GIPS required TWR with monthly periodicity and treatment of external cash flows. From 2010, daily valuation is required for new composites when large cash flows occur. For periods without large cash flows, Modified Dietz approximation remains compliant.

Many legacy systems and smaller firms still use Modified Dietz. Understanding its formula and limitations is essential for CFA Level III performance measurement questions.

What is the Modified Dietz method and when is it used?

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