SungJin Cho/Journal of Economic Research 13 (2008) 125–148
125
The Eﬀectiveness of Parametric
Approximation:A Case of Main
frame Computer Investment
SungJin Cho
1
Seoul National University
Received 15 April 2008;Accepted 14 May 2008
Abstract
The general method to solve the ﬁxed point problem is a dis
cretization of observed state variables.When the observed state
variable is continuous,the required ﬁxed point is in fact an inﬁnite
dimensional object.Therefore,in order to solve the ﬁxed point
problem,it is necessary to discretize the state space so that the
state variable takes on only ﬁnitely many values.But there are
limits regarding this method:(i) “curse of dimensionality”;(ii) the
limits it places on our ability to solve highdimensional DP prob
lems.Despite these limits,this method have been used in many
literature.However,The discretization method may not be appli
cable to computer replacement research to solve the ﬁxed point
problem,because of the aforementioned problems.Using a de
tailed data set on computer holdings by one of the world’s largest
telecommunication companies,this paper shows the eﬀectiveness
of Parametric Approximation procedure by comparison with the
discretization method,which converts the contraction ﬁxedpoint
problem into a nonlinear least squares problem with combining
maximum likelihood estimation method to estimate the unknown
parameters.
Keywords:Parametric Approximation;Discretization;Contin
uous State Variables;Optimal replacement and Upgrade;DP
model;NonlinearNested Fixed Point Algorithm (NLSNFXP)
JEL classiﬁcation:C3,C4
1
Correspondence:SungJin Cho,Assistant Professor,Department of Economics,
Seoul National University (Phone) 8228806371 (Fax) 8228864231 (Email) sung
cho@snu.ac.kr.All errors are my own.
126
The Eﬀectiveness of Parametric Approximation
1 Introduction
The general method to solve the ﬁxed point problem is a discretiza
tion of observed state variables.When the observed state variable is
continuous,the required ﬁxed point is in fact an inﬁnite dimensional
object.Therefore,in order to solve the ﬁxed point problem,it is nec
essary to discretize the state space so that the state variable takes on
only ﬁnitely many values.But there are limits regarding this method:
(i) “curse of dimensionality”;(ii) the limits it places on our ability to
solve highdimensional DP problems.Despite these limits,this method
have been used in many literature.However,The discretization method
may not be applicable to computer replacement research to solve the
ﬁxed point problem,because of the aforementioned problems.In this
paper,using a detailed data set on computer holdings by one of the
world’s largest telecommunication companies,I shows the eﬀectiveness
of Parametric Approximation procedure in comparison with the dis
cretization method,which converts the contraction ﬁxedpoint problem
into a nonlinear least squares problem with combining maximum likeli
hood estimation method to estimate the unknown parameters.
In the “information economy”,comparatively little is known about
the factors aﬀecting investment decisions,including timing of upgrade
and replacement choices.In the face of rapid technological progress and
steadily declining costs,consumers and ﬁrms must decide whether to
upgrade or replace existing computer systems now,or wait to purchase
a faster/cheaper system in the future.According to Moore’s law each
new CPU (Central Processing Unit) contains roughly twice as much
capacity as its predecessor in every 18 months.In the storage industry,
density has been doubling every 12 months,which is faster than the
speed of CPU development.
For example,Figure 1 illustrates that how Moore’s law explains de
veloping trend of computer technology in terms of Intel CPUs.The
time frame of my data starts from 1989 and ends on 1999,where 1M
transistors per CPU (486 DX Processor) has changed to over 24Mtran
sistor CPU (Pentium III Processor) according to Figure 1.In that
period,computer technology had been developed tremendously and the
technological obsolescence is accelerating.Table 1 also presents how
SungJin Cho/Journal of Economic Research 13 (2008) 125–148
127
Figure 1
Note:Data are obtained from Intel.
Moore’s law acts in development of various components of computer
system.We may note that there is a tremendous improvement in com
puter technology between 1984 and 1997.Furthermore,we expect much
faster technological development between 1997 and 2009.
This paper presents a dynamic programming model of a ﬁrm’s de
cision of whether to keep,upgrade,or replace an existing computer
subject to uncertainty in the demand for services and over the timing
and magnitude of future cost reductions of new computer systems.I es
timate this model using a detailed data set on computer holdings of one
of the world’s largest telecommunications companies.An initial analysis
of these data leads to the following conclusions.First,the durations be
tween successive upgrades or replacements have become shorter during
the last two decades,possibly reﬂecting the increased rate of technolog
ical progress in computing equipment during this time period.Second,
computer replacements occurred roughly at a 6year cycle at the be
ginning of the sample period,decreasing to 5year cycle at the end of
the period.These facts support that the presence of rapid technological
progress aﬀects the ﬁrm’s replacement and upgrade policy along with
the economic development.
In section 3.I develop a stochastic dynamic programming model to
128
The Eﬀectiveness of Parametric Approximation
Table 1.Moore’s Law in Action
Year
1979 1984 1997 2009
RAM
16K 128K 12mb 3251mb
Hard Drive
128K 400K 750mb 203,187mb
Speed
2 10 150 40,637
Cost
$5,000 $3,900 $1,400 $10
Note:Data are obtained from Intel.
see whether these stylized facts of replacement and upgrade behavior
can be rationalized as an optimal investment strategy for this ﬁrm.In
the model,the ﬁrmhas three possible actions at each time period:keep,
upgrade,or replace.If replace,there is an array of capacity choices for
a new computer system.The state variables include the processing ca
pacity of the current system,the level of demand for this processing
capacity,the age of the current system,and the current market price of
a standardized unit of processing capacity.Then,I estimate the model
using a nonlinear nested ﬁxed point algorithm (NLSNFXP) incorpo
rating a parametric approximation method to solve the DP problem.
The nonlinear nested ﬁxed point algorithm is a maximum likelihood es
timation,in which outside of maximumlikelihood estimation,the above
nonlinear least square estimation (NLS) is performed to calculate ﬁxed
points and inside of maximum likelihood estimation,based on the NLS,
to estimate unknown parameters.
In section 4,I investigate a parametric approximation procedure,
which greatly reduces the computational burden involved in solving the
inﬁnitehorizon version of the model.The parametric approximation
procedure converts the contraction ﬁxedpoint problem into a nonlinear
least squares problem.I show that this latter problem can be solved
much more rapidly than standard methods based on discretization of
state space.I also show the eﬀectiveness of the parametric approxima
tion method in comparison with a sample result from discretization.
Section 5 investigates a policy implication of the model by deriving
the aggregate demand functions for investment of mainframe computer
systems.Section 6 ﬁnally provides some concluding comments.
SungJin Cho/Journal of Economic Research 13 (2008) 125–148
129
2 Summary of related literature and The Data
Rust (1987)’s seminal work on systems replacement provides a gen
eral template for approaching this topic.In this paper,he formulates a
regenerative optimal stopping model for bus engine replacement to de
scribe the behavior of the superintendent of maintenance at the Madison
Metropolitan Bus Company.In particular,Rust presents that the su
perintendent’s decisionmaking behavior on bus engine replacement can
be implemented as an optimal stopping rule.It is a strategy for decid
ing when to replace current bus engines,and is given as a function of
observed and unobserved state variables.The optimal stopping rule is
formulated as the solution to a stochastic dynamic programming prob
lem that formalizes the tradeoﬀ between the conﬂicting objectives of
minimizing maintenance costs and minimizing unexpected failures of
bus engines.
This paper is important in at least two aspects.First,it provides a
general framework that can be used to analyze replacement behavior in
various ﬁelds.It is the ﬁrst research that uses a “bottom up approach”
for modeling replacement investment.Second,the paper develops a
nested ﬁxedpoint algorithmfor estimating dynamic programming mod
els of discrete choices.The algorithm is very useful in solving problems
that arise typically in investigating replacement behavior.The results
in the paper have been widely applied since its publication,and have
been extended by many authors in numerous directions
2
.
Despite its signiﬁcant role in terms of its estimation techniques,
Rust’s model including Rust’s other papers cannot be applied directly to
the computer replacement research,since Rust’s estimation techniques
focus on the discretization of state variables.When the observed state
variable is continuous,the required ﬁxed point is in fact an inﬁnite di
mensional object.Therefore,in order to solve the ﬁxed point problem,
it is necessary to discretize the state space so that the state variable
takes on only ﬁnitely many values.However,in this computer replace
ment research,the state variables are continuous and thus,the required
ﬁxed points are inﬁnite,which we can’t solve due to the computational
2
aircraft engine mainetnance:Kennet (1994),cement Kilns:Das and Rust,and
nuclear power plants:Rothwell and Rust (1997)
130
The Eﬀectiveness of Parametric Approximation
burden.
Regading the data,I obtained data from one of the biggest telecom
munication companies in the world.It handles over 60 percent of the
entire phone services in the market at which it operates.It also oﬀers
several other telecommunication services,such as cellular PCS (Per
sonal Communications Service),internet,cable,and satellite communi
cation services.The company has 864 hosts (including workstations)
and about 39,000 PCs as of 1998.These hosts and PCs are spread
out in 400 regional headquarters and regional oﬃces.All regional head
quarters operate independently and own their computer systems,even
though there is diﬀerence in terms of capacity.Therefore,in most cases,
each regional headquarter decides maintenance and investment of its
mainframe computers independently.
The computer systems in the company can be divided into two parts
according to use:(i) research use,and (ii) service and management use.
Since computers for research use are purchased and replaced on project
basis,their maintenance activities do not reﬂect technological depreci
ation
3
.Thus,I only consider computer systems for only service and
management use in the data for this research.I also do not include the
replacement of PCs in the company,since in PC replacement there is no
upgrade activity and there only are block purchases and replacements.
There are several tasks within service and management use.Table 1
presents important tasks in service and management use.
The time frame of the data set starts from 1989 and ends on 1999.
The data prior to 1989 are incomplete,though some computer systems
have a history starting from earlier dates,such as 1977,1979,1983,and
1985.Within this time frame (198999),I have a full history of upgrade
and replacement for 123 computer systems in the company.The data
consists of dates of introduction,purchase prices,speciﬁcations,dates
of upgrades and replacements,prices of upgrades and replacements,de
tails of replacements and upgrades,such as system speciﬁcations.The
numbers of customers for services provided by the ﬁrmalso are available
as a form of monthly data.
I divided all computer systems in the sample into two categories
3
Mainframe computers in research use have only ﬁnite horizon bases of research
project,which is diﬀerent from an assumption of the model,an inﬁnite horizon case.
SungJin Cho/Journal of Economic Research 13 (2008) 125–148
131
in terms of the two diﬀerent standards of CPU benchmarks,which are
MIPS (Million Instructions per Second) and TPC (Transaction Pro
cessing performance Council).Currently,the MIPS standard is in the
process of being merged into the TPC standard,which includes the tpm
(transactions per minute) and tps (transactions per second).However,
since my data set consists of various computer systems and dates,it
is very diﬃcult to convert the MIPS standard into the TPC standard.
Within each standard,I divide computer systems into diﬀerent task
groups.Once a certain system brand is designated to serve a given
task,the later replacement is from the same or at least similar sys
tem brand.Table 2 illustrates the diﬀerent groups of major tasks and
number of systems in terms of the two CPU standards.All mainframe
computer systems are associated with speciﬁc tasks.
Tables 3(a),(b),and (c) illustrate several examples of the short
ening of intervals in certain computer systems assigned to major tasks.
One reason for shorter intervals is that the pace of development in the
computer industry has become signiﬁcantly faster and thus the current
system becomes obsolete much more quickly.
3 The Model and Estimation
Suppose that,at every month of the year,a system administrator
investigates the status of each computer system and decides whether to
upgrade,replace,or keep.As a result,the choice set is A
t
= {0,u,1},
where (a
t
= 0) is to keep the system unchanged,(a
t
= u) is an upgrade,
and (a
t
= 1) represents a replacement of system.When the choice is to
replace,the system administrator needs to choose the capacity of the
new system,i.e.,there are n subchoices of capacities,K
1
,...,K
n
.Each
K
r
is a capacity choice for replacement.I assume that two of the state
variables are discrete,which are the capacity and the age of a current
computer system.Two additional variables are continuous,being the
demand for services and the cost per capacity in the computer market.
The state set in the model is x:x
t
= {d
t
,k
t
,g
t
,c
t
},where d
t
=
demand for services,k
t
= current capacity of the computer system,g
t
=
age of each computer system,c
t
= real cost per capacity,which can be
132
The Eﬀectiveness of Parametric Approximation
Table 2.Computers included in the sample in terms of CPU standards
CPU standard
MIPS TPC
Number*
48 57
Tasks
BillingDevelopment Business InfoManagement
BillingManagement Customer Development
General Management Total Document
New Customer Infosystem PreBilling
Super High Speed Printer LineManagement
Material information
Note:* number of computers in the sample.
Table 3(a).Examples of Activities and brands of computers in
various tasks
Task 1
Brand of system*
A B B
Region
1 2 3
New purchase→ﬁrst action**
38 months 37 months 24 months
Interval 1st−→2nd action
23 months 24 months 19 months
Interval 2nd−→3rd action
20 months 11 months 22 months
Interval 3rd−→4th action
17 months 8 months 13 months
Interval 4th−→5th action
15 months 17 months 11 months
Interval 5th−→6th action
12 months 11 months 12 months
Note:* AUnisys system,BHoneywell and Unisys system (MIPS standard),
** Actions includes upgrade and replacement.
SungJin Cho/Journal of Economic Research 13 (2008) 125–148
133
Table 3(b)
Task 2
Brand of system*
C C C C
Region
1 2 3 4
New purchase →ﬁrst action
27 months 18 months 18 months 22 months
Interval 1st −→ 2nd action
25 months 15 months 16 months 18 months
Interval 2nd −→ 3rd action
12 months 12 months 12 months 15 months
Interval 3rd −→4th action
18 months 15 months 11 months 12 months
Interval 4th −→5th action
12 months 13 months 12 months 10 months
Interval 5th −→ 6th action
11 months 1 0months..
Note:* CTandem system (TPC standard).
Table 3(c)
Task 3
Brand of system*
D D D D
Region
1 2 3 4
New purchase →ﬁrst action
59 months 36 months 42 months 51months
Interval 1st −→ 2nd action
23 months 35 months 38 months 34 months
Interval 2nd −→ 3rd action
20 months 15 months 13 months 10 months
Interval 3rd −→4th action
13 months 13 months 9 months.
Note:* DToray and Fujitsu system (MIPS standard).
134
The Eﬀectiveness of Parametric Approximation
seen as a market price of capacity.The two state variables g
t
and k
t
explain internal states of computers and the remaining variables d
t
and
c
t
represent external states of computer systems.
An aggregate demand D
t
consists of the sum of the individual de
mands,d
t
for services provided by each task.The aggregate demand
for services is assumed to follow an AR(1) process.
4
The real cost per capacity,c
t
is bounded by zero and evolves as
c
t+1
=
δ
t
c
t
with 1 − b
c
t
with b
where δ
t
has a truncated normal distri
bution with mean μ and ν
2
with a range of 0 < δ
t
< 1.
The age variable,g
t
represents the age of each computer system.
Since the ﬁrm has the predetermined rule of replacement according to
the age of each system,I intend to keep track of the age of each system.
I assume that each mainframe computer system is speciﬁcally asso
ciated with a certain task.That is,there is only one computer system
per a task.Also,the purchase of additional computers as an alternative
to replacement is prohibited.The proﬁt function for a task consists of
two components,revenue function,R(q(f
t
(k,a
t
),g
t
),d
t
,θ
1
,a
t
) and cost
function,C(f
t
(k,a
t
),g
t
,d
t
,θ
1
,ε).θ
1
is a set of unknown parameters for
proﬁt function.q(f
t
(k,a
t
),g
t
) is an adjusted capacity.q(f
t
(k,a
t
),g
t
)
illustrates how capacity contributes to the revenue function.For exam
ple,the contribution of capacity will decline as a computer gets old.You
can refer the details of proﬁts and cost functions from to Cho (2007).
The optimal value function V
θ
for each task is deﬁned by
V
θ
(x,ε) = max
a∈A
[π(x
t
,a,θ
1
)+ε
t
(a)+β
y
η
V
θ
(y,η
5
)p(dy,dηx
t
,ε
t
,a,θ
0
)]
(1)
Then,we can write the log likelihood function at time t as
l(x
1
,...x
T
,a
1
,...a
T
x
0
,a
0
,θ) =
T
t=1
ln(P(a
t
x
t
,θ)) +
T
t=1
ln(p(x
t
x
t−1
,θ
0
)).
4
ln(D
t
) = a +ρ ln(D
t−1
) +μ
t
withμ˜IIDN(0.
2
)andρ < 1forstationarity..
5
I follow Rust(1987) in making the standard simple assumption that the tran
sition probability η can be factored as ϕ(x
t+1
,ε
t+1
 x
t
,ε
t
,a
t
,θ
0
) = p(x
t+1

x
t
,a
t
,θ
0
)q(ε
t+1
 x
t+1
) where θ
0
is a vector of unknown parameters characterizing
the transition probability for the observable part of the state variables
SungJin Cho/Journal of Economic Research 13 (2008) 125–148
135
You can ﬁnd more details of the model in Cho(2007).
Incorporating the parametric approximation,the estimation requires
the nested ﬁxed point algorithm,which is intended to ﬁnd parameters
that maximize the likelihood functions,subject to the constraint that
function EV
θ
is the unique ﬁxed point.
One of the beneﬁts of a parametric approximation is that discretiza
tion of continuous state variables is no longer required.The two discrete
state variables are still discretized in the manner suggested in section 5.
Additionally,the age variable is discretized in much ﬁner dimension.It
is discretized into months instead of bimonthly cycle.That is,the age
variable ranges from 1 to 60,where the absorbing state,60 represents
that computer system is ﬁve years of age.
The estimation procedure by NLSNFXP is that,i) outside of the
system,θ
0
,parameters for state variables are estimated and simulated
separately from the structural parameters.ii) inside of the system,
θ
1
,the structural parameters are estimated by the nested ﬁxed point
algorithm.That is,outside of maximum likelihood estimation,the
above nonlinear least square estimation (NLS) is performed and ﬁxed
points EV
θ
is calculated.Based on the ﬁxed points,the maximum
likelihood estimation
6
is performed for the the partial log likelihood
function,
T
t=1
ln(P(a
t
x
t
,θ)).For simplicity,I estimate the parameters
θ
0
= {a,ρ,μ,ν,b} which govern the transition probabilities for demand
and cost per capacity separately from the parameters of proﬁts func
tion.The parameters of cost per capacity,c
t
are obtained by maximum
likelihood estimation method.You can refer the detailed estimation
procedure and results from Cho (2007).
4 Parametric Approximation
The general method to solve the ﬁxed point problem is a discretiza
tion of observed state variables.When the observed state variable is
continuous,the required ﬁxed point is in fact an inﬁnite dimensional
6
The Berndt,Hall,Hausman,and Hall (BHHH) alogorithm is used,along with
numerical derivatives.
136
The Eﬀectiveness of Parametric Approximation
object.Therefore,in order to solve the ﬁxed point problem,it is nec
essary to discretize the state space so that the state variable takes on
only ﬁnitely many values.But there are limits regarding this method:
(i) “curse of dimensionality”;(ii) the limits it places on our ability to
solve highdimensional DP problems.Despite these limits,this method
have been used in many literature.
The discretization method may not be applicable to computer re
placement research to solve the ﬁxed point problem,because of the
aforementioned problems.The details are in the following:
The most conservative dimension of a possible combination of state
variables resulting from discretization in the proposed model is 540,000.
Discrete variables,capacity and age are discretized as follows.First,
I discretize the age variable,g
t
,into bimonthly cycle,even though I
have monthly data.Thus,age 1 represents a new computer,
7
and an
absorbing state 30 means 5 years of age.
8
Second,regarding the capacity level,the current data set of the ca
pacity consists of the three elements of CPU,hard drive and memory
size.In order to concretize and transform the capacities into actual
numbers which can represent the capacity of each computer system,I
take a weighted average of these three elements.Since CPU is the most
important factor in the capacity of computer systems,I give it a weight
of 0.5.On the other hand,I give equal weights to Hard Drive and Mem
ory size,namely 0.25.At this time,I do not separate the capacity into
the two standards of CPU benchmark,TPC and MIPS.Even though
the weights were conﬁrmed with the system administrators in the ﬁrm,
their appropriateness will be veriﬁed in further research.With trans
formed capacities of computer systems,I discretize the capacity from 1
to 40.The last state 40 is the absorbing state.Diﬀerence between each
step is 30.Therefore,1 represents (1,...,30),and 2 represents (31,...,60),
and 40 represents the range,(1171,...,+∞).These two discrete variables
should be discretized regardless of the parametric approximation.
The continuous variables,demand and cost per capacity,can be dis
cretized as follows.First,I discretize demand from 1 to 30.Like the
actual capacity,the last state 30 is the absorbing state.Demand 1
7
literally 2 months old.
8
For estimation purpose,I discretize the age variables into months instead of bi
monthly cycle.
SungJin Cho/Journal of Economic Research 13 (2008) 125–148
137
represents 100,000 to 105,000 users and the absorbing state 30 is from
245,001 to ∞users.
Second,I discretize the cost per capacity into 15 possible costs such
as {15,14,...,1}.Diﬀerence between subsequent prices is a 20% price
drop.I restrict maximum price drops in one period to just 2 steps.
These assumptions are based on several research data,computer indus
try databooks,and Moore’s Law
9
.
The transition probability matrices,p(d
t+1
d
t
) and p(c
t+1
c
t
) are in
the appendix.
Therefore,the resulting dimension from the discretization is
540,000 = 30 ×40 ×30 ×15.
First,solving the ﬁxed point problem requires calculation of the ex
pected value function.That is,EV
θ
=
y
η
V
θ
(y,η)p(dy,dηx
t
,ε
t
,a,θ
0
).
Even though the Markov transition probability from discretization is a
sparse matrix,it still requires extensive time to calculate expectation of
value function.Second,the polyalgorithm method by Rust (1987) takes
advantage of the complimentary behavior of the two iterations,which
are a combination of contraction iteration and policy iteration
10
This
algorithm enjoys a substantial reduction in time calculating the ﬁxed
point.However,it is not applicable to solving a dynamic programing
model.The reason is as follows:One must have a Frechet derivative
(I −T
θ
) in order to use policy iteration method
11
.But,the dimension
ality problemmakes it impossible to get the derivatives of T
θ
.Thus,the
algorithm for the DP problem consists solely of a backward induction,
which is simple but takes more time to solve.Therefore,the extended
time caused by the two aforementioned reasons seriously aﬀects the cal
9
The index was created by the informations from SIA (Semiconductor Industry
Association)’s annual databooks,the 8th Annual Computer Industry Almanac,ZD
net.com,and Cnet.com.
10
Newton Kantorovich method.
11
The idea of the policy iteration method,i.e.,the Newton Kantorovich iteration
is to ﬁnd a zero solution of the nonlinear operator F = (I −T
θ
) instead of ﬁnding a
ﬁxed point EV
θ
= T
θ
(EV
θ
).With invertibility of (I −T
θ
) and existence of a Frechet
derivative (I −T
θ
),one can do a following Taylor expansion:
0 = [I −T
θ
](EV
l
)˜[I −T
θ
](EV
l−1
) +[I −T
θ
](EV
l
−EV
l−1
).
=⇒EV
l
= EV
l−1
−[I −T
θ
]
−1
(I −T
θ
)(EV
l−1
).
138
The Eﬀectiveness of Parametric Approximation
culation time of a nested ﬁxed point algorithm,because the nested ﬁxed
point algorithm uses the ﬁxed point algorithm outside of the maximum
likelihood estimation.
To begin with,one needs functional forms for the three value func
tions,keep,upgrade,and replacement.To ﬁnd the parametric forms of
value functions,I use the simple linear OLS estimations,such as
V (a = 0,x) = H(x,λ
0
) +ψ
0
(2)
V (a = U,x) = H(x,λ
U
) +ψ
U
V (a = K
r
,x) = H(x,λ
K
r
) +ψ
K
r
where H(x,λ
0
),H(x,λ
U
) and H(x,λ
K
r
) are ﬂexible functions and linear
in λ.ψ
0
,ψ
U
,and ψ
K
r
are assumed to be distributed as N(0,1)
First,I choose the best functional forms for each value function
according to the criteria,
R
2
.After extended search for the appro
priate functional forms of the three value functions,I have the fol
lowing results.
V (a = 0,x) has 12 parameters ( = λ
0
) with 0.983 of
R
2
,
V (a = U,x) has 15 parameters ( = λ
U
) with 0.962 of
R
2
and
V (a = (K
1
...K
n
),x) has 18 parameters ( = λ
K
r
) with 0.962 of
R
2
.
Second,with the approximated functional forms of the three value
functions,I estimate all 45 parameters (λ
0,
λ
U
,λ
K
r
) with nonlinear least
square estimation,such as
min
λ
0
,λ
U
,λ
K
r
j
a
[V
a
(x
j
) −U
a
]
2
(3)
where
U
1
= [({u(x
t
,a
t
= 0,θ
1
)
+β
σ
y
log
a
∈A(y)
exp[V
α
(y)/σ]
p(dyx
t
,a
t
= 0,θ
0
)})]
and
U
2
= [({u(x
t
,a = U,θ
1
)
+β
y
σ log
a
∈A(y)
exp[V
α
(y)/σ]
p(dyx
t
,a
t
= U,θ
0
)})]
and
U
3
= [({u(x
t
,a = K
r
))
+β
y
σ log
a
∈A(y)
exp[V
α
(y)/σ]
p(dyx
t
,a
t
= K
r
,θ
0
)})].
12
12
The above three expectations are calculated by a quadrature method
SungJin Cho/Journal of Economic Research 13 (2008) 125–148
139
Figure 2.Diﬀerences between parametric approximation and
discretization
Solving the above minimization problem enables us to estimate all
parameters
ˆ
λ
0
,
ˆ
λ
U
,and
ˆ
λ
K
r
.In fact,a parametric approximation proce
dure converts the contraction ﬁxedpoint problem into a nonlinear least
squares problem.
4.0.1 Parametric approximation and discretization:A Com
parison
Based on the estimated parameters of the model,I calculate a ﬁxed
point by a discretization method.Comparisons between two value func
tions from discretization and parametric approximation are illustrated
in Figures 2,3,and 4,which represent cases of keep,upgrade,and re
placement,respectively.In each Figure,graph (a) presents the expected
140
The Eﬀectiveness of Parametric Approximation
Figure 3.Diﬀerences between parametric approximation and
discretization
SungJin Cho/Journal of Economic Research 13 (2008) 125–148
141
Figure 4.Diﬀerences between parametric approximation and
discretization
142
The Eﬀectiveness of Parametric Approximation
value function
13
by discretization,and graph (b) shows the diﬀerences
between two value functions from parametric approximation and dis
cretization.Though there is a slight discrepancy in comparisons be
tween two methods,the diﬀerences seem to be negligible.Therefore,
the parametric approximation solves the proposed DP model as accu
rately as and more eﬃciently by speeding up the solution time than a
discretization method does.
The forgoing empirical results lead to two main conclusions:(i) along
with the nested ﬁxed point algorithm,the parametric approximation
method can be a practical,eﬃcient and numerically stable method for
estimating certain structural model lacking closedform solutions with
high dimensional state space.(ii) the data are by and large consistent
with the prediction of the proposed optimal stopping model of main
frame computers replacement and upgrade.In the following section,
interesting behavioral implications of the model will be explored for the
purpose of wide application of the model.
5 Policy Experiments
Even though the model is simple,it leads to a wealth of interesting
behavioral implications.In particular,the model can be used to perform
a wide variety of “policy experiments” which forecast how changes in
various environments of the model,τ
u
and τ
r
or structural parameters,
such as γ and β aﬀect the timing and frequency of mainframe computers
replacement and upgrade.
In order to ﬁnd out the behavior of upgrade and replacement de
mand with respect to an average unit price of service the ﬁrm provides,
γ,again,ﬁrst I should compute the expected upgrade and replacement
demand functions,d
u
(γ) = E{
d
u
(γ)} and d
r
(γ) = E{
d
r
(γ)} respec
tively.Let P
∗
θ
(a,x) be the longrun equilibrium distribution of the
controlled process {a
t
,x
t
}.With an additional assumption of indepen
dency
14
which I mentioned earlier,I can get the following formula for
13
In order to graph value functions in terms of the demand and the capacity vari
ables,I ﬁx the cost per capacity and the age of computer at certain values,such as
relatively high cost per capacity and a fairly new computer.
14
The processes {a
m
t
,x
m
t
} and
a
k
t
,x
k
t
are independent if m
= k
SungJin Cho/Journal of Economic Research 13 (2008) 125–148
143
d
u
(γ) and d
r
(γ) such as,
d
u
(γ) = TM
∞
0
P
∗
θ
(dx,U) (4)
d
r
(γ) = TM
∞
0
P
∗
θ
(dx,K
r
) (5)
where T is the number of month and M is the number of computers.
By varying γ,we can plot upgrade and replacement demand curves
with respect to unit price of service demand.Figure 5 and 6 summarize
the resulting upgrade (d
u
(γ)) and replacement (d
r
(γ)) demand curves
for TPC and MIPS standards respectively.As expected,all demand
functions show upward sloping curves.For both Figures,expected up
grade demand curves (d
u
(γ)) increase with an increasing rate at a lower
part of average unit price and become ﬂat at higher prices.In case
of replacement demands (d
r
(γ)),the behavior is diﬀerent from d
u
(γ).
i.e.,demand increases smoothly at a lower part of price and becomes
steeper at higher average prices.This is because the price is not enough
to induce the replacement behavior at a lower unit prices and as price
increases,replacement which requires much higher cost than upgrade
does is more preferable because higher prices can cover a larger part of
replacement costs
15
.Higher price can also induce better quality services
of the ﬁrm which requires newer and faster computer systems in order
for the ﬁrm to maintain the current,high priced demand.
Note that the estimated equilibrium price,γ equals to 3.167.Until
less than twice of the equilibrium price,upgrade demand lies above the
replacement demand.After the price,replacement demand becomes
larger than upgrade demand.There shows the same phenomenon in
case of MIPS standard computer systems in Figure 6,where the equi
librium γ equals to 3.524.Therefore,we can conjecture that an average
price should be at least twice as much as the equilibrium price in order
to have more likelihoods of replacement than upgrade in this case.
Comparison between two Figures,5 and 6 gives the following inter
esting points.First,when upgrade demand lies above that of replace
ment,the gaps between replacement and upgrade curves at each Figure
15
Replacement is the better choice than upgrade in order to improve a computer’s
performance,when the other conditions remain the same.
144
The Eﬀectiveness of Parametric Approximation
Figure 5.Expected demand curves of upgrade and replacement
(TPC standard computers)
are diﬀerent from each other.The gap in TPC standard is narrower
than that of MIPS standard.This is because the diﬀerence between
upgrade and replacement costs in MIPS standard computers is larger
than that of TPC standard computers.Therefore,an average demand
price in case of MIPS standard should be much higher than that of TPC
standard in order for replacement demand to exceed upgrade demand
16
.
Second,at any given unit prices of service demand,the upgrade de
mand for MIPS standard computers is larger than the upgrade demand
for TPC standard,because the upgrade cost is smaller in case of MIPS
standard.Similarly,the replacement demand for MIPS standard com
puters is smaller than the replacement demand for TPC standard,since
the average replacement cost of MIPS standard is larger than that of
TPC standard computers.As a result,the above simple policy implica
tions show us that how the ﬁrmcan modify its upgrade and replacement
policies to deal with various environments.
16
As mentioned above,the replacement demand exceeds the upgrade demand at
about γ = 7.2 in case of MIPS standard and at about γ = 6.4 in case of TPC
standard according to ﬁgures 5 and 6.
SungJin Cho/Journal of Economic Research 13 (2008) 125–148
145
Figure 6.Expected demand curves of upgrade and replacement
(MIPS standard computers)
6 Conclusion
The paper explains about the Parametric Approximation procedure
and shows its eﬀectiveness when we apply to a high dimensional ﬁxed
point problem.The parametric approximation is used to circumvent
problems,such as curse of dimensionality and computational burden
incurred by discretization and substitutes a nonlinear least squares es
timation method for a ﬁxed point iteration.The speed up in solution
time is suﬃciently large to make it feasible to estimate the unknown
parameters of the model by maximum likelihood.The comparisons be
tween sample results of discretization and those of parametric approx
imation method show the eﬀectiveness and viability of the parametric
approximation in general.The resulting estimation results conﬁrm that
the proposed model can explain replacement and upgrade behavior rea
sonably.Apolicy experiment using the estimated model is accomplished
to show the versatility of the model.
146
The Eﬀectiveness of Parametric Approximation
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The Eﬀectiveness of Parametric Approximation
7 Appendix A
7.1 Transition matrices of demand and cost per capacity
for discretization purpose.
p(D
t+1
D
t
) is as follows.
1 2 3...29 30
1 h
∗
0
1 −h
0
0...0 0
2 h
1
h
0
1 −h
0
−h
1
...0 0
3 0 h
2
h
0
...0 0
...0 0 0.........
29 0 0...0 h
0
1 −h
0
30 0 0...0 0 1
*:
h
0
= 0.89.
p(pc
t+1
pc
t
) is as follows.
15 14 13...2 1
15 g
∗
0
g
∗∗
1
1 −g
0
−g
1
...0 0
14 0 g
0
g
1
1 −g
0
−g
1
0 0
13 0 0 g
0
...0 0
.........0...0 0
2 0 0.......g
0
1 −g
0
1 0 0...0 0 1
*:
g
0
= 0.28,
**:
g
1
= 0.7
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