Panel Unit Root Tests
Pesaran (2007), the Cross-Sectionally Augmented Dickey-Fuller (CADF) unit root test, is one of the most popular unit root tests in the literature and takes cross-sectionally dependent into account. This test is based on the Im et al. (2003) (IPS) modified t-test and the ADF unit root test applied to each unit. In addition, the unit root process of the panel can be examined using the Cross-Sectionally Augmented Panel Unit-Root (CIPS) statistic, which is the average of the t-tests of the units. However, this test needs to take structural change into account.
Many tests are employed to investigate the unit root process of a series under structural change. The described tests figure out changes in the structure under a shift break using dummy variables (Im et al. 2005; Bai and Carrion-i-Silvestre 2009). In contrast to these studies, Lee et al. (2016) added a new unit root test to the body of literature by capturing structural change using the Enders and Lee (2012) Fourier approach. Fourier functions offer a convenient approximation for breaks of unknown form. The most important reason for this is identifying the appropriate component in the model. Therefore, it eliminates complex procedures such as determining break dates, the number of breaks, and the shape of the breaks (Lee et al. 2016).
CIPS test created by Pesaran et al. (2013) is enhanced by adding a Fourier function to the Lee et al. (2016) unit root test to analyze the unit root process. The Break and cross-sectionally dependent Augmented ADF Statistic (BCADF) is an extension of the CADF statistic proposed in Eq. 1 by Lee et al. (2016)
$${\varDelta y}_{it}={c}_{i,0}+{c}_{i,1}sinsin \left(\frac{2\pi kt}{T}\right) +{c}_{i,2}coscos \left(\frac{2\pi kt}{T}\right) +{c}_{\text{1,3}}^{{\prime }}{\underset{\_}{z}}_{t-1}+{c}_{\text{1,4}}^{{\prime }}\varDelta {\underset{\_}{z}}_{t}+{\sum }_{j=1}^{p}{c}_{\text{1,5}}^{{\prime }}\varDelta {\underset{\_}{z}}_{t-1}+{\sum }_{j=1}^{p}{c}_{i,6}\varDelta {y}_{i,t-1}+{b}_{,}{y}_{i,t-1}+{e}_{it}$$
1
In this equation, \(p\) is the optimal lag length, t is the trend value\(, \pi =3.1416\), \(T\) is the sample size, \(k\) is the optimal number of frequencies of the Fourier functions, and \({z}_{it}={\left({y}_{it},{x}_{it}^{{\prime }}\right)}^{{\prime }}\) is the vector of the common factor \({x}_{it}^{{\prime }}\). The Break Augmented CIPS (BCIPS) test statistic developed by Lee et al. (2016) by taking into account the Breaks and Cross-Sectionally Dependent Augmented version of the unit root test under Pesaran (2007) and Pesaran et al. (2013) studies are calculated as follows:
$$BCIPS \left(N, T\right)=\frac{1}{N}{\sum }_{i=1}^{N}{t}_{i}\left(N,T\right)$$
2
where \({t}_{i}\) denotes the test statistic calculated for each unit. In addition, the test’s null hypothesis expresses the unit root process.
Panel Fourier Toda-Yamamoto Causality
The Granger causality test is one of the pioneering tests among causality tests. This test was developed by Granger (1969) and is based on the Vector Autoregressive Model (VAR), but has some limitations. The standard Granger causality analysis requires testing for unit roots and cointegration. This is because if the variables in the VAR model are integrated or cointegrated, the Wald test has a non-standard distribution and depends on nuisance parameters (Yilanci and Gorus 2020; Durusu-Cifci et al. 2020). Toda and Yamamoto (1995) overcame this problem with the approach developed. This approach is based on the VAR \(\left(p+{d}_{max}\right)\) model. In this case, the only thing that has to be calculated is the maximum stationarity level of the variables (\({d}_{max}\)). Using the Fisher test statistic, Emirmahmutoglu and Kose (2011) produced it to apply this causality test to panel data. Emirmahmutoglu and Kose (2011) suggest the bootstrap simulations to obtain the critical values of Fisher test statistics in cross-sectionally dependent.
When structural change is not considered, the results obtained may be misleading. Enders and Jones (2016) demonstrate that the traditional dummy variable approach to modeling structural changes in the VAR framework is inappropriate and propose a new approach based on formulating a Fourier function for structural changes. Therefore, the number, dates, and structure of breaks can be determined using Fourier functions without doing it in advance. Nazlioglu et al. (2016) modified the Toda-Yamamoto model using the Fourier technique and created a new test considering structural changes. Yilanci and Gorus (2020) have extended the Fourier Toda-Yamamoto causality test proposed by Nazlioglu et al. (2016) for panel analysis. In this context, the Panel Fourier Toda-Yamamoto causality test for the two variables is expressed as follows:
$${y}_{i,t}= {\mu }_{i}+{A}_{11}sin\left(\frac{2\pi t{f}_{i}}{T}\right)+{A}_{12}cos\left(\frac{2\pi t{f}_{i}}{T}\right)+{\sum }_{j=1}^{{p}_{i}+{d}_{{max}_{i}}}{A}_{13}{y}_{i,t-j}+{\sum }_{j=1}^{{p}_{i}+{d}_{{max}_{i}}}{A}_{14}{x}_{i,t-j}+{u}_{1i,t}$$
3
$${x}_{i,t}= {\mu }_{i}+{A}_{21}sin\left(\frac{2\pi t{f}_{i}}{T}\right)+{A}_{22}cos\left(\frac{2\pi t{f}_{i}}{T}\right)+{\sum }_{j=1}^{{p}_{i}+{d}_{{max}_{i}}}{A}_{23}{x}_{i,t-j}+{\sum }_{j=1}^{{p}_{i}+{d}_{{max}_{i}}}{A}_{24}{y}_{i,t-j}+{u}_{2i,t}$$
4
where \(\pi =3.1416\); \(t\), trend; \(T\), sample size; \({f}_{i}\), the integer value of the minimum sum of the residual squares of the equations for each unit in the range of 1–5. At the same time, the Fisher test statistic for the Panel Fourier Toda-Yamamoto causality test is calculated as \(FTYP=-2ln\left({p}^{*}\right).\)Here \({p}^{*}\) indicates the Wald statistic calculated by bootstrap technique for each unit.
In the Toda-Yamamoto model, the Wald statistic for the null hypothesis of Granger non-causality follows an asymptotic χ2 distribution with p degrees of freedom (Durusu-Cifci et al. 2020). This test has some advantages. The first one is that it deals with cross-sectionally dependent with the bootstrap technique and takes heterogeneity into account. Second, it uses Fourier functions to account for structural changes in the VAR model. Lastly, it employs the Toda-Yamamoto method to prevent the loss of information about the variables.
The Panel Fourier Toda-Yamamoto causality test analyzes variables without dividing them into positive and negative shocks. However, the variables' impact on one another may vary when they are divided into hidden shocks and examined. For the first time in the literature, Granger and Yoon (2002) analyzed the relationship between variables by using their cumulative positive and negative shocks. For the panel causality test, the cumulative positive and negative shocks for each of the variables are calculated as follows:
$${y}_{i,t}={y}_{i,t-1}+{\epsilon }_{1,i,t}={y}_{i,t,0}+{\sum }_{i=1}^{t}{\epsilon }_{1,ij}$$
5
$${x}_{i,t}={x}_{i,t-1}+{\epsilon }_{2,i,t}={x}_{i,t,0}+{\sum }_{i=1}^{t}{\epsilon }_{2,i,j}$$
6
In equations (5) and (6), \(t=\text{1,2},...,T\); \(\epsilon\) is the error term with white noise. Positive and negative shocks are represented as follows.
$${\epsilon }_{1,i,t}^{+}=max({\epsilon }_{1,i,t}, 0) \text{v}\text{e} {\epsilon }_{1,i,t}^{-}=min \left({\epsilon }_{1,i,t}, 0\right)$$
7
$${\epsilon }_{2,i,t}^{+}=max({\epsilon }_{2,i,t}, 0) \text{v}\text{e} {\epsilon }_{2,i,t}^{-}=min \left({\epsilon }_{2,i,t}, 0\right)$$
8
Error terms are defined as \({\epsilon }_{1,i,t}={\epsilon }_{1,i,t}^{+}+{\epsilon }_{1,i,t}^{-}\) ve \({\epsilon }_{2,i,t}={\epsilon }_{2,i,t}^{+}+{\epsilon }_{2,i,t}^{-}\). Hence, the cumulative positive and negative shocks (9) and (10) calculated for each of the variables can be written as follows by adding equations (7) and (8).
$${y}_{i,t}={y}_{i,t-1}+{\epsilon }_{1,i,t}={y}_{i,t,0}+{\sum }_{i=1}^{t}{\epsilon }_{1,i,t}^{+}+{\sum }_{i=1}^{t}{\epsilon }_{1,i,t}^{-}$$
9
$${x}_{i,t}={x}_{i,t-1}+{\epsilon }_{2,i,t}={x}_{i,t,0}+{\sum }_{i=1}^{t}{\epsilon }_{2,i,t}^{+}+{\sum }_{i=1}^{t}{\epsilon }_{2,i,t}^{-}$$
10
where \({\sum }_{i=1}^{t}{\epsilon }_{1,i,t}^{+}\) and \({\sum }_{i=1}^{t}{\epsilon }_{1,i,t}^{-}\) are respectively positive and negative shocks of \({y}_{i,t}\); \({\sum }_{i=1}^{t}{\epsilon }_{2,i,t}^{+}\) and \({\sum }_{i=1}^{t}{\epsilon }_{2,i,t}^{-}\) are respectively positive and negative shocks of \({x}_{i,t}\). Therefore, the Panel Asymmetric Fourier Toda-Yamamoto causality relationship between variables can be tested as follows by Eq. (11, 12, 13, 14):
$${y}_{i,t}^{+}= {\mu }_{i}+{A}_{11}sin\left(\frac{2\pi t{f}_{i}}{T}\right)+{A}_{12}cos\left(\frac{2\pi t{f}_{i}}{T}\right)+{\sum }_{j=1}^{{p}_{i}+{d}_{{max}_{i}}}{A}_{13}{y}_{i,t-j}^{+}+{\sum }_{j=1}^{{p}_{i}+{d}_{{max}_{i}}}{A}_{14}{x}_{i,t-j}^{+}+{u}_{1i,t}$$
11
$${x}_{i,t}^{+}= {\mu }_{i}+{A}_{21}sin\left(\frac{2\pi t{f}_{i}}{T}\right)+{A}_{22}cos\left(\frac{2\pi t{f}_{i}}{T}\right)+{\sum }_{j=1}^{{p}_{i}+{d}_{{max}_{i}}}{A}_{23}{x}_{i,t-j}^{+}+{\sum }_{j=1}^{{p}_{i}+{d}_{{max}_{i}}}{A}_{24}{y}_{i,t-j}^{+}+{u}_{2i,t}$$
12
$${y}_{i,t}^{-}= {\mu }_{i}+{A}_{11}sin\left(\frac{2\pi t{f}_{i}}{T}\right)+{A}_{12}cos\left(\frac{2\pi t{f}_{i}}{T}\right)+{\sum }_{j=1}^{{p}_{i}+{d}_{{max}_{i}}}{A}_{13}{y}_{i,t-j}^{-}+{\sum }_{j=1}^{{p}_{i}+{d}_{{max}_{i}}}{A}_{14}{x}_{i,t-j}^{-}+{u}_{1i,t}$$
13
$${x}_{i,t}^{-}= {\mu }_{i}+{A}_{21}sin\left(\frac{2\pi t{f}_{i}}{T}\right)+{A}_{22}cos\left(\frac{2\pi t{f}_{i}}{T}\right)+{\sum }_{j=1}^{{p}_{i}+{d}_{{max}_{i}}}{A}_{23}{x}_{i,t-j}^{-}+{\sum }_{j=1}^{{p}_{i}+{d}_{{max}_{i}}}{A}_{24}{y}_{i,t-j}^{-}+{u}_{2i,t}$$
14
The Data
This study aims to examine the relationship between foreign direct investment inflows and outflows and pollution levels in European Union countries (Austria, Belgium, Bulgaria, Cyprus, Czechia, Denmark, Finland, France, Germany, Greece, Hungary, Ireland, Italy, Malta, the Netherlands, Poland, Portugal, Romania, the Slovak Republic, Spain, and Sweden). For this objective, the share of foreign direct investment inflows (FDIN) and outflows (FDOUT) in national income (GDP%) is taken into account. Furthermore, CO2 emission (kt) has been considered since, according to the European Commission (2020) report, carbon dioxide emissions are identified as the gas that causes the most pollution among greenhouse gases. All data is obtained from the World Bank (2023). The study covers between 1993–2019, in which the countries' data is jointly available. Additionally, since the relevant data for 27 European Union countries could not be obtained, 21 countries were included in the study. Descriptive statistics for the variables are given in Table 2.
Table 2
| CO2 | FDIN | FDOUT |
Mean | 150371.6 | 11.286 | 7.795 |
Median | 65750 | 2.968 | 1.735 |
Maximum | 904340 | 449.081 | 300.406 |
Minimum | 1350 | -40.087 | -87.226 |
Std. Dev. | 185480.3 | 38.216 | 32.867 |
This study examines the relationship between foreign investments and carbon dioxide emissions for each country by employing Panel Fourier Toda-Yamamoto tests for both symmetric and asymmetric causality in bidirectional models (CO2 = f(FDIN), FDIN = f(CO2), CO2 = f(FDOUT), FDOUT = f(CO2)). Developed countries typically conduct foreign investments (United Nations Conference on Trade and Development, 2021). Since the European Union consists mainly of developed countries, similar countries engage in foreign investment in these countries. Since these countries have strict environmental policies, FDOUTs are anticipated to impact pollution more than FDINs substantially. This is because countries generally relocate their dirty industries to less developed countries.