4.3. Robustness Test
Furthermore, we have incorporated robustness tests into our analysis to ensure that the results are reliable and valid, even when data deviate from ideal conditions or violate statistical assumptions. These tests bolster our confidence in the consistency of our primary findings by reassuring us that they are not artifacts of particular model specifications or data characteristics.
(1) Parallel trend test
A critical assumption for policy evaluation using the DID model is satisfying the parallel trend condition. This implies that in the absence of the NBDCPZ implementation, the GTFP change trends between the control group and treatment group should show no significant differences. To verify this assumption, this study employs event analysis following Jacobson et al. (1993) [
60]. As shown in Formula (2),
is a series of dummy variables, which denote whether the NBDCPZ pilot policy was implemented in year
k in city
c.
is the difference between the control group and treatment group at year k. If any of
is significant during
, it indicates that the control group and treatment group fulfill the parallel trend assumption prior to the policy’s implementation. This study chooses the first period before the pilot as the base period.
Figure 5 shows the 95% confidence intervals for the dynamic effect’s estimated coefficients, which are not significant in the years before the pilot implementation. This reveals that the GTFPs of the treatment and control groups have a relatively similar change trend before the pilot, thus satisfying the assumption. Furthermore, for the first and fourth periods after the pilot, the estimated coefficients are significantly positive. These demonstrate that the construction of NBDCPZ does, indeed, have a substantial promotional influence on GTFP.
(2) Placebo test
In order to exclude the impact of other random variables, this study executes an individual placebo test and a time placebo test by selecting treatment groups at random. This involves randomly selecting cities in our sample as pilot zones for the NBDCPZ, while preserving the same data distribution, to construct fictional treatment and control groups for regression analysis. If no interference from other random effects exists, the estimated coefficients corresponding to these fictitious policy dummy variables should be insignificantly distinct from zero. This suggests that the random selection of pilot cities has no significant effect on urban GTFP. To ensure the robustness of our findings, we repeat this random selection process 500 times via bootstrapping.
Figure 6 (left) shows the kernel densities of the estimated coefficients and
p-value distributions. The distributed coefficients randomly cluster around zero, with their corresponding
p-values basically bigger than 0.1. Meanwhile, the coefficient of the real policy dummy variable DID is 0.2291, marked by the dashed line in
Figure 6, and it is obviously different from those obtained in the placebo test. Additionally, this study conducts a time-point placebo test with the generated kernels of estimated coefficients and their
p-value distributions plotted in
Figure 6 (right). Similar to the individual placebo test, the distributed coefficients randomly cluster around zero, with their corresponding
p-values basically bigger than 0.1. Therefore, we conclude that random factors do not disrupt the NBDCPZ’s influence on GTFP. The policy effect of the NBDCPZ is not a coincidence, which bolsters the robustness of our baseline conclusions.
(3) PSM-DID
This study incorporates the propensity score matching (PSM) method to alleviate the endogeneity problem caused by the potential sample selection bias. The primary aim of this method is to select control groups. Here, we pick control groups from non-pilot cities, using the previously used control variables in regression as covariates. Next, we use the logit model to estimate the propensity score values. By employing radius matching with a radius of 0.05 for each period, the cities with the closest scores are chosen as the matched control group for the pilot cities of the NBDCPZ. The balance test shows that the standardized deviation of all covariates is less than 10%, and the t-value is not significant. This indicates that the standardized deviations of the covariates with significant differences have been significantly reduced, implying that all covariates pass the balance test. After obtaining the new control groups, we re-examine the relationship between NBDCPZ and GTFP. In
Table 3, columns (1) and (2), similar to the baseline regression results, the core explanatory variable DID is significantly positive. Moreover, to ensure the robustness of the results, we use the period-by-period 1:4 nearest-neighbor matching method. In
Table 3, columns (3) and (4), the core explanatory variable DID is still significantly positive. After alleviating potential sample selection bias and self-selection bias, this study employs the PSM-DID method to obtain consistent estimation results, further demonstrating that the research conclusions are not influenced by specific samples, and the conclusions remain robust.
(4) Instrumental Variable (IV) Approach
In order to mitigate the endogeneity problems caused by simultaneity bias and omitted variable bias, this study uses the instrumental variable (IV) approach. Taking cues from studies by Barone et al. (2015) [
61] and Tian et al. (2022) [
62], we use the distance from each city to the nodal city of “Eight Verticals and Eight Horizontals” fiber optic trunk network as our instrumental variable. In the past few decades, China has built up an “Eight Verticals and Eight Horizontals” fiber optic trunk network that covers all provincial capital cities and key areas across the nation. From the standpoint of correlation, the “Eight Verticals and Eight Horizontals” fiber optic trunk network forms the basic framework of China’s communication network and lays the foundation for subsequent regional digital infrastructure development. From an exogenous standpoint, the impact of the “Eight Verticals and Eight Horizontals” fiber optic trunk network project in history has little impact on green development. Therefore, it meets the exogenous requirements. Our instrumental variable regression results, represented in
Table 4, reveal that after satisfying the weak instrument test, the pilot policies of the NBDCPZ continue to have a significantly positive impact on urban GTFP growth. In column (2), the estimated coefficient of DID is 0.1834 and is significant at the 5% level. This indicates that the policy shock of NBDCPZ has led to a 0.1834% increase in GTFP in pilot cities relative to non-pilot cities. The instrumented DID coefficient (0.1843, Column (2) of
Table 4) is somewhat smaller than the OLS estimate (0.2074, Column (2) of
Table 2), suggesting that the endogenous explanatory variable leads to overestimating the impact of the NBDCPZ on GTFP. However, we also deduce that the construction of the NBDCPZ robustly fosters urban GTFP escalation.
(5) Treatment effect heterogeneity
Considering that the policy’s pilot project was implemented in two separate phases, potential biases might be present. This is because the staggered difference-in-differences estimation is essentially a weighted average of varying treatment effects. In instances where negative weights occur, the weighted average of treatment effects may significantly diverge from the true average treatment effect. Therefore, the Goodman-Bacon decomposition method is implemented (Goodman-Bacon, 2021) [
63] to assess the degree of bias within DID estimations. The results of this decomposition, as illustrated in
Figure 7, indicate that an appropriate weighting of treatment effects results in a remarkably high convergence rate of 99.96%. This lends further support to the robustness and reliability of our findings.
Additionally, using the CSDID method provided by Callaway and Sant’ Anna (2021) [
64], the average estimated treatment effect calculated through aggregation is shown in
Table 5. The treatment group’s average treatment effects (ATT) are significantly positive at a 5% confidence level, demonstrating that the research findings remain robust even after mitigating the heterogeneity in treatment effects.
(6) Other robustness tests
Several robustness tests were implemented in this study as follows: ① Outlier removal: The dependent variable was winsorized at the 1% and 99% percentiles to mitigate the influence of extreme values. As
Table 6, column (1) shows, the direction and significance of the core explanatory variable (DID) remained consistent with the baseline regression results. ② Substituting the dependent variable: We changed the input–output variable combination, and only PM2.5 was selected as the non-desired output to measure GTFP. As
Table 6, column (2) shows, the coefficient on the core explanatory variable (DID) remained significantly positive. ③ Excluding other policy effects: We selected the “Broadband China” program, conducted in three batches from 2014 to 2016, to exclude the competitive effects of other policies on urban GTFP. As
Table 6, column (3) shows, the coefficient on the core explanatory variable (DID) remained significantly positive. ④ Excluding specific cities: The sample excludes four municipalities and a number of provincial capitals. As
Table 6, column (4) shows, the regression results remained robust. ⑤ Adjusting the timing of the pilot implementation: The times of the pilot implementation for all cities in Guizhou Province were set to 2016. As
Table 6, column (5) shows, these findings collectively underscore that our baseline regression results retain their robustness across multiple test conditions performed in this study. ⑥ Introducing regional fixed effects: This study further controls for regional fixed effects. As
Table 6, column (6) shows, the coefficient of the core explanatory variable (DID) remained significantly positive.
4.4. Heterogeneity Analysis
The impact of big data on GTFP could differ among various cities. This study conducts heterogeneity analysis based on three aspects: industrial foundation, resource foundation, and urban scale.
Firstly, according to the industrial foundation, we categorize our sample cities into traditional industrial cities and other cities. Traditional industrial cities refer to the industrial cities that were established in the early stages of China’s national development, with a focus on heavy industry backbone enterprises. In
Table 7, columns (1) and (2), the results show that in the traditional industrial cities, urban digital transformation significantly increases GTFP, while the effect is not significant for other cities. A possible reason is that the impact of urban digital transformation on GTFP requires a developed industrial system as support.
Secondly, according to the resource bases, we categorize our sample cities into resource-based cities and other cities. Resource-based cities are mainly dominated by local mineral, forest, and other natural resources and mining and processing industries. In
Table 7, columns (3) and (4), the results show that for resource-based cities, the effect of urban digital transformation on GTFP is not significant, whereas other cities experience a significant improvement in GTFP through digital transformation. This can be attributed to the fact that resource-based cities rely heavily on natural resources, thereby diminishing the impact of urban digital transformation on GTFP. Meanwhile, the other cities are more likely to break away from traditional development paths and are inclined to increase GTFP through dedicated efforts in digital transformation.
Thirdly, according to the urban scale, we categorize our sample cities into large cities and small cities. Large cities are defined as those with a permanent population of over 500,000, while small cities have a permanent population of below 500,000. In columns (5) and (6) of
Table 7, the results show that for large cities, the effect of urban digital transformation on GTFP is not significant, whereas in small cities, digital transformation significantly improves GTFP. The possible reason might be that small cities selected for the NBDCPZ have a stronger desire for digital transformation, resulting in more investment in elements such as digital capital and digital technology. Consequently, the pilot project of the NBDCPZ has a more significant effect on improving GTFP.
This section presented the empirical evidence of this study, as well as a number of robustness checks, and the results show that the construction of the NBDCPZ increases GTFP. The next section will carry out the mediating effects test.