## 3 Materials and Methods  
Based on major events that have occurred during the process of urbanization in Mongolia, for example, the land reforms (the \"Law on Allocation of Land to Mongolian Citizens for Ownership\" was implemented in 2003 [10], and the number of rural migrants entering Ulaanbaatar reached its peak in 2004 [7]. And then the Mongolian congress extended the validity of the \"land use right\" for up to another five years in 2013 [10]), the dzud disaster (the dzud disaster from 2009 to 2010 forced hundreds of nomads to move to cities [27]), and economic events (the Central Bank of Mongolia raised the benchmark interest rate to address the problem of inflation and the reduction of workers' relative income in 2016 ([[http://pit.ifeng.com/a/20160903/49892274_0.shtml](http://pit.ifeng.com/a/20160903/49892274_0.shtml)]([http://pit.ifeng.com/a/20160903/49892274_0.shtml](http://pit.ifeng.com/a/20160903/49892274_0.shtml))))), we selected data in the five periods of 2004, 2007, 2010, 2013, and 2016 to analyze the CCD between urbanization and residents' livelihood level. The main data sources were the Statistical Yearbooks of Mongolia and the National Statistical Bureau of Mongolia ([[http://www.1212.mn/en/](http://www.1212.mn/en/)]([http://www.1212.mn/en/](http://www.1212.mn/en/))). We also collected firsthand data by interviewing local administration officials, academic scholars, and rural migrants. The Indexes Used for the Evaluation of the Level of Urbanization and the Livedihood Level of Urban Residents  
Previous research on urbanization has empirically used a single index, such as the population index [37], the land index [38, 39], and the population density index [40], to measure the level of urbanization. The level of urbanization is the quantitative value of the degree of urbanization that relies on the connotation of urbanization. As mentioned above, the process of urbanization involves many aspects, such as the development of the economy and society, rural immigration, and land expansion [4]. Thus, composite indices such as the City Development Indicators [41] and the Urban Indicators Guidelines [42], which generally include population, economic, social, and other indicators [19, 43], are more universally suitable for measuring the level of urbanization. Accordingly, we constructed composite indexes to evaluate the level of urbanization based on the City Development Indicators and the Urban Indicator Guidelines. The evaluation index system that was used in this study comprised three dimensions: A population index, an economic index, and a social index. Significantly, in addition to the population and economic indexes, traffic facilities and solid waste disposal are important indicators used to measure the level of urbanization [44]. The final evaluation index system is shown in Table 1, and the weight of each indicator was calculated through the entropy weight method using the steps shown in Section 3.2. Figure 1: Urbanization and the population density of Mongolia. Source: The map is from the Mongolia National Statistical Bureau ([[http://www.1212.mn/en/](http://www.1212.mn/en/)]([http://www.1212.mn/en/](http://www.1212.mn/en/))), and the population and urbanization data are from the Mongolia Statistical Yearbooks. Previous research on the livelihood level of residents originated in the United States in the 1930s [46], and was concentrated in the fields of medicine, psychology, and sociology. Since the 1970s, scholars have focused more on the application of \"quality of life\" research, particularly emphasizing the evaluation of livelihood level [47]. Some scholars have measured the livelihood level of residents from the two subjective aspects of \"satisfaction\" and \"happiness\" based on the subjective dimension of individual experience [48; 49].