## 2 Theoretical Background
For the purposes of this study, i.e., a literature overview, research studies focused on the efficiency and level of service in airport passenger terminals were selected with the aim of showing the diversity of approaches. In 2006, [PERSON] and [PERSON] [29] applied the AHP method to assess the security factor of ASQ and concluded that human factors and their behavior are crucial factors. The aim of this study was to examine the importance of different factors in improving passenger security control process at an airport. Using AHP analysis based on the collected survey data on the relative importance of factors related to improving passenger security control, it was determined that human resources are the most important factor in increasing security control efficiency. This result is in line with the globally accepted views in the air transport industry. In the paper of [PERSON] et al. [30], in 2007, the AHP method was applied to determine the importance that users give to various components of an airport passenger terminal (attributes of an APT) and their characteristics, such as processing time and staff courtesy. A survey of departing passengers at Sao Paulo/Guarulhos International Airport, the largest airport in South America, reveals that for departing passengers, the most important APT components are the check-in counter and the gate holdrooms. [PERSON] et al. [31], in 2011, proposed a gap analysis using a hybrid framework for decision-making at an airport in Taiwan. This paper aims to develop a multi-criteria evaluation model to assess the gap between the passenger's perception (perceived quality of service) and his/her expectation (expected quality of service) and to define management strategies to reduce the gap in the context of airport passenger services. This multi-criteria evaluation model is combined with the AHP method, the VIKOR method and the Importance-Performance Analysis (IPA) technique. The application of AHP and VIKOR methods to empirical data was used to analyze passenger preferences and passenger satisfaction, as well as to show the gap between users of airport services. The IPA technique was then applied to diagnose managerial strategies to reduce the gap between passenger perception and expectation. The authors of [31] developed an integrated model, which includes various questions for the evaluation of airport services provided to passengers. In 2012, [PERSON] and [PERSON] [32] presented an integrated robust DEA-AHP model to measure the relative efficiency of different facilities within a passenger terminal (e.g., check-in, security control, and immigration control). The authors conclude that the proposed model can provide more accurate efficiency results than the standalone application of DEA or AHP methods. Published in 2015, the paper by [PERSON] et al. [33] deals with increasing the efficiency of an airport in terms of transfer passengers. The aim of the research is to implement measures to increase the volume of transfer passengers at Incheon International Airport using AHP. In order to achieve the objective, this study conducted a survey to evaluate various criteria for selecting transfer airports and ranked attributes using AHP. Furthermore, based on the analysis of the transfer passenger movement trends of Incheon International Airport, this paper analyzes the main transit routes of Incheon International Airport compared to competing airports in the region. The main result of the study is that flight frequency, price,  
Figure 1: Criteria and sub-criteria for ASQ enablers in airport passenger terminals [28]. and connection time are the three most important factors when people choose transfer airports. Therefore, it is recommended that the government, airport operators and airlines work closely together to meet the needs of transfer passengers in terms of the frequency, price, and connection time of transfer flights. The main goal of the research from 2015 by [PERSON] et al.