
SECTION
10.1
Comparing Mutually Exclusive Alternatives
is
assumed to be equal for all alternatives. Public sector projects normally
are service-based
wi
th
the difference between costs and timing used to select
one alternative over another.
Recommended method: Whether an analysis
is
performed by hand
or
by com-
puter, the methodes) recommended in Table
10-1 will correctly select one
alternative from two or more as rapidly as possible. Any other method can
be applied subsequently to obtain additional information and,
if
needed,
verification
of
the selection. For example,
if
lives are unequal and the rate
of
return
is
needed, it
is
best to first apply theAW method at the MARR and
then determine the selected alternative's
i*
using the same
AW
relation
with
i as the unknown.
Series to evaluate: The estimated cash flow series for one alternative and the in-
cremental series between two alternati ves are the only two options for present
worth
or
annual worth evaluation. For spreadsheet analyses, this means that
the
NPY or PY functions (for present worth) or the PMT function (for annual
worth)
is
applied. The word "updated"
is
added
as
a remainder that a study
period analysis requires that cash flow estimates (especially salvage/market
values) be reexamined and updated before the analysis is performed.
Once the evaluation method is selected, a specific procedure must be followed.
These procedures were the primary topics
of
the last five chapters. Table 10-2
summarizes the important elements
of
the procedure for each
method-PW,
AW,
ROR, and B/C.
FW
is included
as
an extension
ofPW.
The meaning
of
the entries
in Table
10-2 follows.
Equivalence relation: The basic equation written to perform any analysis is
either a
PW or an
AW
relation.
The
capitalized cost (CC) relation is a PW
relation for infinite life, and the
FW
relation
is
likely determined from the
PW equivalent value. Additionally, as we learned in Chapter 6,
AW
is
simply
PW
times the
AI
P factor over the
LCM
of
their lives.
Lives
of
alternatives and Time period
for
analysis: The length
of
time for an
evaluation (the
n value) will always be one
of
the following: equal lives
of
the alternatives,
LCM
of
unequal lives, specified study period, or infinity
because the lives are so long.
PW analysis always requires the LCM
of
all alternatives.
Incremental
ROR and
B/C
methods require the LCM
of
the two
alternatives being compared.
The A W method allows analysis over the respective alternative lives.
The one exception is for incremental ROR method for unequal-life alterna-
tives using
an
AW
relation for incremental cashflows. The
LCM
of
the two
alternatives compared must be used. This is equivalent to using an
AW
relation for the actual cash flows over the respective lives. Both approaches
find the incremental rate
of
return
t:..i*.
Series to evaluate: Either the estimated cash flow series or the incremental series
is
used to determine the PW value, the A W value, the
i*
value, or the B/C ratio.
349
I Sec.
6.1
I
1
How
PW, A W,
and
FW
relate