5.1.1. The DID Results

The city fixed effect, year fixed effect, and city time trend effect variables are gradually added in Model 1, and the standard errors are clustered at the city level. Compared with other regions, the industrial energy intensity may have some inherent change trend rather than the BTH coordinated development strategy effect. Ignoring the underlying trend change of dependent variables in the treatment group will produce bias of missing variables and make the estimated results unreliable. The interaction item between city and time trends is added in column (2) of Table 1. This interaction item control for the likely trends in the treatment group. Please note that, in this paper, all subsequent regressions are controlled for the city and year fixed effects and there may be no repeated reporting in the subsequent tables.



Source: Authors' estimation. Notes: *p*-values are in brackets \*\*\* *p* < 0.01.

In Table 1, the BTH coordinated development strategy has a significantly negative impact on industrial energy intensity in this region. Listed in columns (1) and (2), the coefficients of the interaction terms are significantly negative at the 1% level. It shows that the BTH coordinated development strategy tends to restrain industrial energy intensity in this region. In addition, due to the extensive mode of rural ecological management, insufficient investment in environmental protection infrastructure, and technical limitations in Hebei Province, the rural ecological environment has been seriously polluted and destroyed, especially for the water source polluted by industrial wastewater and the soil polluted by solid waste. Therefore, attention should be paid to relevant environmental law enforcement efforts in the rural areas of this region.

As for the control variables, the regression results of FDI stock over GDP are negative but not statistically significant, indicating that FDI had no significant influence on industrial energy intensity. This is consistent with previous literature [42]. The regression results of the other control variables are consistent with theories and expectations.
