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{{Short description|Historical performance of an investment portfolio}}
The '''modified Dietz method'''<ref name="Dietz1966">{{cite book
|author=Peter O. Dietz
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|url=https://books.google.com/books?id=ZUQ_MwEACAAJ
|year=1966
|publisher=Free Press}}</ref><ref>{{Cite journal|last=Dietz|first=Peter|date=May 1968|title=Measurement of Performance of Security Portfolios COMPONENTS OF A MEASUREMENT MODEL: RATE OF RETURN, RISK, AND TIMING|journal=The Journal of Finance|volume=Volume 23, Issue 2|issue=2|pages=267–275|doi=10.1111/j.1540-6261.1968.tb00802.x}}</ref><ref name="CIPMCFA2009">{{cite book▼
|publisher=Free Press}}</ref><ref name="CIPMCFA2009">{{cite book
|author1=Philip Lawton, CIPM
|author2=Todd Jankowski, CFA
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|isbn=978-0-470-47371-9
|pages=828–
|quote=Peter O. Dietz published his seminal work, Pension Funds: Measuring Investment Performance, in 1966. The Bank Administration Institute (BAI), a U.S.-based organization serving the financial services industry, subsequently formulated rate-of-return calculation guidelines based on Dietz's work.}}</ref> is a measure of the ''ex post'' (i.e. historical) performance of an [[investment portfolio]] in the presence of external flows. (External flows are movements of value such as transfers of cash, securities or other instruments in or out of the portfolio, with no equal simultaneous movement of value in the opposite direction, and which are not income from the investments in the portfolio, such as interest, coupons or dividends.)
 
To calculate the modified Dietz return, divide the gain or loss in value, net of external flows, by the average capital over the period of measurement. The average capital weights individual cash flows by the length of time between those cash flows until the end of the period. Flows which occur towards the beginning of the period have a higher weight than flows occurring towards the end. The result of the calculation is expressed as a percentage [[rate of return|return]] over the holding period.
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|publisher=Institute of Chartered Financial Analysts.
|page=72
|quote=A slightly improved version of this method is the day-weighted, or modified Dietz, method. This method adjusts the cash flow by a factor that corresponds to the amount of time between the cash flow and the beginning of the period.}}</ref> The original idea behind the work of Peter Dietz was to find a quicker, less computer-intensive way of calculating an IRR as the iterative approach using the then -quite -slow computers that were available was taking a significant amount of time; the research was produced for BAI, Bank Administration institute.{{Citation needed|date=October 2017}} The modified Dietz method is a linear [[Internal_rate_of_return|IRR]].
 
His approximation was therefore to generate money weighted rates of return for the period. Because there is a GIPS requirement to produce a valuation on a monthly basis at least, using modified Dietz with monthly valuations provides a series of individual monthly money-weighted rates with which can be compounded together to produce a good quality approximation for the longer time period time weighted rate of return<ref>{{Cite journal|last=Dietz|first=Peter|date=May 1968|title=Measurement of Performance of Security Portfolios
▲COMPONENTS OF A MEASUREMENT MODEL: RATE OF RETURN, RISK, AND TIMING|journal=The Journal of Finance|volume=Volume 23, Issue 2|issue=2|pages=267–275|doi=10.1111/j.1540-6261.1968.tb00802.x}}</ref>
 
==Formula==
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==Comparison with time-weighted return and internal rate of return==
The modified Dietz method has the practical advantage over the [[true time-weighted rate of return]] method, in that the calculation of a modified Dietz return does not require portfolio valuations at each point in time whenever an external flow occurs. The [[internal rate of return]] method shares this practical advantage with the modified Dietz method.
Conversely, if there exists a portfolio valuation at any point in time, the implied modified Dietz valuation of cashflows at that point in time is quite unlikely to agree with the actual valuation.
 
With the advance of technology, most systems can calculate a true time-weighted return by calculating a daily return and geometrically linking in order to get a monthly, quarterly, annual or any other period return. However, the modified Dietz method remains useful for performance attribution, because it still has the advantage of allowing modified Dietz returns on assets to be combined with weights in a portfolio, calculated according to average invested capital, and the weighted average gives the modified Dietz return on the portfolio. Time weighted returns do not allow this.
 
The modified Dietz method also has the practical advantage over [[internal rate of return]] (IRR) method that it does not require repeated trial and error to get a result.<ref name="Feibel2003">{{cite book
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The (unannualized) internal rate of return in this example is 125%:
 
:<math>300 = 100 \times (1 + 125\%)+ 50 \times (1+125\%)^ \frac{2 - 1}{2} = 225 + 50 \times 150\% = 225 + 75 = 300</math>
 
so in this case, the modified Dietz return is noticeably less than the unannualized IRR. This divergence between the modified Dietz return and the unannualized internal rate of return is due to a significant flow within the period, andtogether with the fact that the returns are large. If there are no flows, there is no difference between the modified Dietz return, the unannualized IRR, or any other method of calculating the holding period return. If the flows are small, or if the returns themselves are small, then the difference between the modified Dietz return and the unannualized internal rate of return is small.
 
The IRR is 50% since:
 
:<math>100 \times (1 + 50\%)^2 + 50 \times (1+50\%)^ 1 = 225 + 50 \times 150\% = 225 + 75 = 300</math>
 
but the unannualized holding period return, using the IRR method, is 125%. Compounding an annual rate of 50% over two periods gives a holding period return of 125%:
 
:<math>(1 + 50\%)^2 - 1 = 2.25 - 1 = 1.25 = 125\%</math>
 
==The simple Dietz method==
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There are no flows, so the gain or loss is:
 
:{{nowrap begin}}{{link if exists|end value}} -− {{link if exists|start value}} = 1,125,990 -− 1,128,728 = -2−2,738 {{link if exists| HKD}}{{nowrap end}}
 
and the average capital equals the start value, so the modified Dietz return is:
 
:{{nowrap begin}}{{sfrac|{{link if exists|gain or loss}}|{{link if exists|average capital}}}} = {{sfrac|-2−2,738|1,128,728}} = -0−0.24 % 2 d.p.{{nowrap end}}
 
===Contributions - when not to adjust the holding period===
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====Example====
Suppose that at the beginning of the year, a portfolio contains cash, of value $10,000, in an account which bears interest without any charges. At the beginning of the fourth quarter, $8,000 of that cash is invested in some US dollar shares (in company X). The investor applies a buy-and-hold strategy, and there are no further transactions for the remainder of the year. At the end of the year, the shares have increased in value by 10% to $8,800, and $100 interest is capitalized into the cash account.
 
What is the return on the portfolio over the year? What are the contributions from the cash account and the shares? Furthermore, what is the return on the cash account?
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:{{nowrap begin}}{{link if exists|average capital}}{{nowrap end}}
::{{nowrap begin}}= {{link if exists|start value}} -− {{link if exists|time weight}} &times; {{link if exists|outflow amount}}{{nowrap end}}
::{{nowrap begin}}= 10,000 -− {{sfrac|1|4}} &times; $8,000{{nowrap end}}
::{{nowrap begin}}= 10,000 -− $2,000{{nowrap end}}
::{{nowrap begin}}= $8,000{{nowrap end}}
 
The average capital of the shares over the last quarter requires no calculation, because there are no flows after the beginning of the last quarter. It is the $8,000 invested in the shares. However, the average capital in the shares over the whole year is something else. The start value of the shares at the beginning of the year was zero, and there was an inflow of $8,000 at the beginning of the last quarter, so:
:{{nowrap begin}}{{link if exists|average capital}}{{nowrap end}}
::{{nowrap begin}}= {{link if exists|start value}} -− {{link if exists|time weight}} &times; {{link if exists|outflow amount}}{{nowrap end}}
::{{nowrap begin}}= 0 + {{sfrac|1|4}} &times; $8,000{{nowrap end}}
::{{nowrap begin}}= $2,000{{nowrap end}}
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==Money-weighted return==
The modified Dietz method is an example of a money (or dollar) weighted methodology (as opposed to [[time-weighted return|time-weighted]]). In particular, if the modified Dietz return on two portfolios are <math>R_1</math> and <math>R_2</math>, measured over a common matching time interval, then the modified Dietz return on the two portfolios put together over the same time interval is the weighted average of the two returns:
 
:<math>W_1 \times R_1+W_2 \times R_2</math>
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There are sometimes other difficulties when decomposing portfolio returns, if all transactions are treated as occurring at a single point during the day.
 
For example, consider a fund opening with just $100 of a single stock that is sold for $110 during the day. During the same day, another stock is purchased for $110, closing with a value of $120. The returns on each stock are 10% and 120/110 -− 1 = 9.0909% (4 d.p.) and the portfolio return is 20%. The asset weights ''w<sub>i</sub>'' (as opposed to the time weights ''W<sub>i</sub>'') required to get the returns for these two assets to roll up to the portfolio return are 1200% for the first stock and a negative 1100% for the second:
 
:w*10/100 + (1- − w)*10/110 = 20/100 → w = 12.
 
Such weights are absurd, because the second stock is not held short.
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:At the end of Day 40, the remaining 20 shares are worth 12.50 dollars per share
 
The gain or loss is end value -− start value + outflow:
 
:<math>20 \times 12.50 - 100 \times 10 + 80 \times 15</math>
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:<math>\frac {\text {gain or loss}}{\text {average capital}} = \frac {450}{-50} = -900 \%</math>
 
Instead, we notice that the start value is positive, but the average capital is negative. Furthermore, there is no short sale. In other words, at all times, the number of shares held is positive.
 
We then measure the simple return from the shares sold:
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and combine these returns with the weights of these two portions of the shares within the starting position, which are:
 
:<math>\frac {80}{100} = 80 \%</math> and <math>\frac {20}{100} = 20 \%</math> respectively.
 
This gives the contributions to the overall return, which are:
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====Limitations====
This workaround has limitations. It is possible only if the holdings can be split up like this.
 
It is not ideal, for two further reasons, which are that it does not cover all cases, and it is inconsistent with the Modified Dietz method. Combined with Modified Dietz contributions for other assets, the sum of constituent contributions will fail to add up to the overall return.
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==Visual Basic==
 
<sourcesyntaxhighlight lang="vbvbnet">
Function georet_MD(myDates, myReturns, FlowMap, scaler)
' This function calculates the modified Dietz return of a time series
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End Function
</syntaxhighlight>
</source>
 
==Java method for modified Dietz return==
<sourcesyntaxhighlight lang="java">
private static double modifiedDietz (double emv, double bmv, double cashFlow[], int numCD, int numD[]) {
 
/* emv: Ending Market Value
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try {
 
double[] weight = new double[cashFlow.length];
 
if (numCD <= 0) {
throw new ArithmeticException ("numCD <= 0");
}
 
for (int i = 0; i < cashFlow.length; i++) {
if (numD[i] < 0) {
throw new ArithmeticException ("numD[i]<0 , " + "i=" + i);
}
weight[i] = (double) (numCD - numD[i]) / numCD;
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double ttwcf = 0; // total time weighted cash flows
for (int i = 0; i < cashFlow.length; i++) {
ttwcf += weight[i] * cashFlow[i];
}
 
double tncf = 0; // total net cash flows
for (int i = 0; i < cashFlow.length; i++) {
tncf += cashFlow[i];
}
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return md;
}
</syntaxhighlight>
</source>
 
== Excel VBA function for modified Dietz return ==
<syntaxhighlight lang="VBVBnet">
Public Function MDIETZ(dStartValue As Double, dEndValue As Double, iPeriod As Integer, rCash As Range, rDays As Range) As Double
 
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