2.2. Methodology
The main purpose of this research is to investigate and analyze the spatial difference of urban energy consumption in China with regard to the 30 provincial capital cities with the support of Geographical Information Systems (GIS) tools and Theil index. The first step involved the adoption of GIS mapping technology to represent urban energy consumption indicators in spatial form involving the 30 case cities of mainland China. GIS methods offer the opportunity to direct viewing the spatial difference associated with their consumption pattern on map, in addition to direct comparison of values of the related indicators. Then Theil index for each group was calculated to quantitatively demonstrate these spatial variances from the perspective of energy consumption scale, efficiency and structure of concerned case cities. Compared to other indicators as Gini index, Mean deviation and Atkinson index, the Theil index is an appealing inequality index measuring regional differences, since it can be easily broken down into two synthetic components, i.e., within-group inequality component and between-group inequality component [11]. Alcantara and Duro proposed the use of Theil index to analyze international energy intensity differences among OECD countries, and found that the OECD countries reduced their differences in terms of energy intensity between 1971 and 1999 [12]. In this paper, the basic Theil index can be defined as follow: