*4.1. Data and Variables*

This paper uses the DID method to investigate the impacts of the BTH coordinated development strategy on industrial energy intensity and related pollution emission intensities in this region by comparing the implementation of this strategy before and after between the treatment group and the control group [31]. Industrial energy intensity and the pollution emission intensity are defined as the ratios of industrial energy consumption and industrial pollution emissions to the value-added of the industry.

This paper uses panel data, containing 30 cities from 2007 to 2017, which includes all 13 cities in the BTH region and 17 cities covering all cities of Henan Province. Henan is chosen as the control group because it is adjacent to the BTH region and they are similar in industrial energy structure, the sources of industrial energy pollution, and their variation trends. Another reason is that, except for Henan, other provinces around the BTH region have only a few recent years of statistics on industrial energy consumption and do not cover the years before implementation of the BTH coordinated development strategy. Although some southern Chinese provinces have related data, the statistical indicators for industrial pollution emissions are inconsistent with the BTH region and Henan province and the structures, especially for the variation trends of industrial energy and pollution emissions, are obviously far from that of the BTH region. The data of industrial fossil energy consumption and industrial value-added come from each Provincial Statistic Yearbook [32–35]. The data of industrial energy-related pollution emissions and all control variables come from the China City Statistical Yearbooks [36].

According to previous studies, all the models control for the following variables, which are expected to have impacts on industrial energy intensity and related pollution intensities in the BTH region. Urbanization is defined as the proportion of urban population in the total population and is denoted as *UR* [37], the industrial structure is defined as the ratio of service industrial value-added over GDP and is denoted as *SER* [38], per capita GDP is the natural logarithm itself and is denoted as *PGDP* [39], foreign direct investment (FDI) is defined as the share of FDI stock over GDP and is denoted as *FDI*/*GDP* [40], and R&D is defined as the share of the R&D expenditure over GDP and is denoted as *RD* [41].
