INTRODUCTION
The MundellFleming (MF) model has been significant in much of the professional
work on open macroeconomics since Mundell (1962, 1963)
and Fleming (1962). In 1999, Nobel Prize in economics
was awarded to the MF model. The MF model is an extension of the ISLM model
in that it includes a demonstration of how the exchange rate and the flows of
net exports are determined. MF model tries to clarify the responses of a small
open economy and the limits in which it runs in a world of free capital movements.
One of the key assumptions of MF model is that the Balance Of Payments (BOP)
is zero in equilibrium; that is, the trade balance equals the net capital outflow.
However, so far enough attention is hardly paid on the characteristic of BOP
time series. If BOP is mean reverting, it follows that the BOP will return to
its trend path over time and the assumption of MF model is reasonable. On the
other hand, if BOP follows a random walk process, any shock or innovation has
a sustained effect. Thus, the future BOP cannot be predicted based on its historical
movements and requires a modification in the analytical approach of MF model
(Lin and Wang, 2008).
To the best of our knowledge, the work of Lin and Wang
(2008) is the first attempt to introduce whether BOP series are mean reverting
or not. Using the quarterly observations for G7 countries from 1981:1 to 2006:3
period and performing the standard ADF unit root tests, they show that the BOP
may follow a random walk process in some series. However, for most of the G7
countries, the BOP is trend stationary when the unit root tests are allowing
for one structural break in the trend function. This implies that the assumption
of MF model about the equilibrium of BOP is appropriate.
The purpose of this study is to investigate whether BOP is mean reverting or not in OECD countries via panel unit root methodology. Considering the aforementioned MF model, this study is designed to contribute to the validity of MF model’s key assumption by testing whether the balance of payments (BOP) is zero in equilibrium for OECD countries. What distinguishes this study from the previous ones is that Panel unit root tests are employed for the first time.
MATERIALS AND METHODS
The data set, comprises annual observations for 24 OECD countries among 30 OECD countries, is obtained from the International Monetary Fund’s International Financial Statistics (IFS).
The sample period ranges from 1982 to 2007. In order to measure the equilibrium
of BOP, two indexes, BOP1 and BOP2 are employed. BOP1 is obtained by summing
up the net balance in current account, capital account and financial account.
Allowing for the statistical discrepancy, the other index, BOP2 is defined as
overall balance which adds the net errors and omissions to the previous BOP1.
All the accounts in BOP series are measured in billions of US dollars. BOP1 and BOP2 series are deflated by the consumer price index of each country.
It is well known that traditional unit root tests possess low power against
near unit root alternatives (Diebold and Nerlove, 1990).
A popular test for verifying unit roots is the Augmented DickeyFuller (ADF)
test in which the null hypothesis is nonstationarity. However, these statistics
are applied to time series data sets. Therefore, the most effectual choice is
the application of panel unit root test. The pioneer of the panel unit root
is Abuaf and Jorion (1990). In an influential study
of Abuaf and Jorion (1990) developed a multivariate
unit root test based on systems estimation of autoregressive processes for a
set of real exchange rate series and use this to reject the joint null hypothesis
of nonstationarity of a number of real exchange rates.
First generation unit root tests (crosssectional independence)
Im, Pesaran and Shin’s panel unit root test: The Im
et al. (2003) IPS hereafter is based on the traditional augmented
Dickey Fuller specification:
IPS allows for a heterogeneous coefficient of y_{it1} and proposes a testing procedure based on averaging individual unit root test statistics.
The null hypothesis is given by the existence of a unit root in all the units of the panel against the alternative of at least one stationary crosssection.
To test the hypothesis, IPS proposes a standardized tbar statistic given by:
Levin, Lin and Chu ttest: The LLC test is carried out by estimating the following equation:
The panel OLS of the normalized residuals is run to obtain the β estimates. And LLC show that under the null hypothesis H_{0}: β = 0, the regression tstatistic (t_{β}) has a standard normal limiting distribution.
Second generation unit root tests (crosssectional dependence): After
the study of Abuaf and Jorion (1990),
Levin and Lin (1992), O’Connell (1998) and
Sarno and Taylor (1998) the panel unit root tests was
improved by considering crosssectional correlation. O’Connell
(1998) was the first author to note that crosssectional correlation in
panel data will have negative effects on the LevinLin panel unit root test,
making the test have substantial size distortion and low power.
Kristian (2005) studied the performance of the LevinLin
test under crosssectional correlation. In his DGP (Data Generation Processes),
he controlled the magnitude of the correlation and found results similar to
the results of O’Connell (1998).
Pesaran (2005) proposed the simple averages of the individual
crosssectional augmented ADF statistics (CIPS), based on a single common factor
specification for the crosscorrelation structure. Simulation results under
the assumption of a single common factor and known autocorrelation order of
the residuals demonstrate that the CIPS test performs very well. The Pesaran
tests augment the standard ADF regressions with the cross section averages of
lagged levels and first differences of the individual series (CADF).
The CADF tests which assume serial correlated errors are based on the tstatistics of the OLS estimate of β_{1} in the following regression:
where, t_{i}(N,T) the tstatistic of the coefficient of y_{i, t1} in the CADF regression for the ith countries.
RESULTS AND DISCUSSION
Both IPS and LLC unit root test results illustrate that BOP is mean reverting,
it follows that the BOP will return to its trend path overtime and the assumption
of MundellFleming model is reasonable (Table 1).
The first generation of tests includes Levin et al.
(2002) and Im et al. (2003), which are all
constructed under the assumption that the individual time series in the panel
are crosssectionally independently distributed.
Table 1: 
First generation unit root tests 

*99% confidence level. To decide the number of lags, Modified
Schwarz Information Criterion (MSIC) is used 
Table 2: 
Second generation unit root test 

The distribution of the CIPS test is nonstandard and the
critical values for 1, 5 and 10% have been tabulated by Pesaran
(2005) for different combinations of N and T. The critical values for
the intercept case for 10, 5 and 1% significance levels are 2,070, 2.150
and 2.300, respectively. The critical values for the intercept and trend
case for 10, 5 and 1% significance levels are 2.580, 2.660 and 2.810, respectively.
And to decide the number of lags, Modified Schwarz Information Criterion
(MSIC) is used 
On the contrary a large amount
of literature provides evidence of the comovements between economic variables. To overcome this difficulty, a second generation of tests rejecting the crosssectional
independence hypothesis has been suggested.
In addition to the first generation unit root tests results with crosssectional
independence, if this survey considers the crosssectional correlation by CIPS
test, Table 2 displays similar results with IPS and LLC tests.
Therefore, it is concluded that BOP is mean reverting, it follows that the BOP
will return to its trend path overtime and the assumption of MundellFleming
model is reasonable. Results confirm the study of Lin and
Wang (2008). They also concluded that the BOP is trend stationary when the
unit root tests are allowing for one structural break in the trend function.
This implies that the assumption of MundellFleming model about the equilibrium
of BOP is appropriate for G7 countries.
CONCLUSION
The MundellFleming model has been significant in much of the professional work on open macro economics. One of the key assumptions of MundellFleming model is that the balance of payments (BOP) is zero in equilibrium; that is, the trade balance equals the net capital outflow. This study is devoted to explore to see whether BOP is mean reverting in OECD countries or not. The results obtained in this research which are based on first and second generation panel unit root techniques indicate that BOP follows a stationary process for 24 OECD countries. These findings suggest that the assumption of MundellFleming model is reasonable.