332 Performance Measurement in Finance
The numerator still yields the net amount of money realized while the denom-
inator gives us the amount of money that was available to the manager for
the period.
This approach only prices the portfolio at the start and end of the period.
To enhance the return’s accuracy, we should revalue whenever a flow occurs.
The true daily rate of return formula
ROR =
EMV
1
BMV
1
+
EMV
2
BMV
2
+···+
EMV
n
BMV
n
(12.6)
is intended to produce an accurate rate of return, devoid of the affects of flows
or market volatility.
Each individual fraction demarcates the periods from one flow to the next,
starting with the beginning period market value and ending with the period’s
ending value. The tricky part is determining whether or not the flow occurred
at the start or end of the day. But we’ll discuss this further below.
There are other methods available, such as the modified BAI, ICAA and unit
value formulae. Space does not permit going into these in detail. However, the
ones described above are the most commonly used ones and will be discussed
further below.
12.3 CONTRASTING THE METHODS
12.3.1 The scenario we’ll use
We’ll use the following example to contrast the various approaches and to
demonstrate the importance of properly handling cash flows:
31 May: End-of-month market value = $100,000
4 June: End-of-day market value = $100,500
5 June: Cash flow of $500,000
End-of-day market value (without cash flow) = $130,500
End-of-day market value (with cash flow included) = $630,500
30 June: End-of-month market value = $640,000
The portfolio ended the month of May with a market value of $100,000.
During the month of June, a single (but very large) cash flow of $500,000
occurred; this happened on 5 June. The market value from the preceding
day was $100,500 (i.e. the portfolio had increased by $500 from the start of
month). At the end of 5 June, there was an appreciation of $30,000. If we
add this amount to the start-of-day value, we get $130,500; if we add it to
the start-of-day plus the cash flow, we get $630,500. The market value at the
end of June was $640,000.
The rate-of-return formula can make a difference 333
12.3.2 Mid-point Dietz
We’ll begin by using the ‘mid-point Dietz’ method. It assumes that all cash
flows occur at the middle of the period. As noted earlier, the formula is pretty
straightforward:
ROR
MPD
=
EMV BMV C
BMV +0.5 ×C
(12.7)
where:
ROR
MPD
= the rate of return, using the mid-point Dietz method
EMV = the ending period market value
BMV = the beginning period market value
C = the sum of the cash flows for the period.
The ‘0.5’ in the denominator is what causes the cash flows to be treated as if
they occurred at the middle of the period.
If we substitute the values from above, we get:
ROR
MPD
=
640,000 100,000 500,000
100,000 +0.5 × 500,000
=
40,000
350,000
= 11.43%
(12.8)
The advantage of this method is its simplicity. It requires you to calculate
only two market values: the starting and ending. And you simply sum all the
cash flows and treat them as if they occurred in the middle.
While some people have used this method in the past for periods as long
as a calendar quarter or even a full year, it’s been more appropriate of late to
use it for calculating monthly returns. It is, however, also suitable for shorter
periods.
The main problem with this approach is that it treats flows as if they
occurred in the middle of the period. This is obviously not always the case.
The accuracy of this approach diminishes when (1) the actual flow date moves
farther away from the middle of the month and (2) when the size of the flow,
relative to the starting market value, is large.
Traditionally, the industry has accepted ‘large’ to mean 10% or more of the
market value.
7
In our case, a cash flow of $10,000 (10% of $100,000) would
qualify as large. Therefore, a flow of $500,000 would have to be described
as very large. And, given that the actual flow date was 5 June, a whole
10 days earlier than the mid-point of 15 June, we’d expect the accuracy to be
questionable.
7
This is one of those ‘unwritten’ rules that has become a de facto standard.

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