## 4 Discussion  
As mentioned in several of the previous literature surveys [8], one limitation of the current IEQ model development is the need for more unified data collection and analysis methods. To increase the accuracy and applicability of the IEQ models, a standardized IEQ measurement and analysis procedure is needed. The final models might be different for the different types of occupants and the various kinds of buildings. However, the data collection techniques and analysis methods used should be integrated and standardized to build a universal scientific database that can continuously expand. A series of comprehensive weighting schemes can be developed for all types of buildings. Then, targeted and specific measures could be suggested to improve IEQ accordingly. Consistent data collection protocols are needed to achieve this goal, whereby IEQ indicators and the related temporal and spatial measurement strategies are clearly stated. A standardized questionnaire should also be provided. A widely applicable data analysis process with step-by-step instructions is also necessary. Moreover, considering the interaction effects between the IEQ aspects/indicators, all the potential IEQ influencing factors should be investigated together, and a unified, integrated data analysis method should be used. Furthermore, previous studies found that different occupants respond differently to the same indoor environment because people differ; therefore, not all are satisfied by the same conditions. A field study conducted among more than 1000 Dutch schoolchildren identified six different types of schoolchildren, based on their different perceptions, needs, and preferences for IEQ in classrooms: 'concerned with sound', 'concerned with smell- and sound', 'concerned with thermal temperatures and drafts', 'concerned with light levels', 'all concerned cluster', and 'nothing concerning' [28]. Additionally, the subjects' perceived importance of IEQ aspects changed according to the occupants' gender [38], profession [41], education level [7], and the type of building wherein they were occupied [17]. Therefore, further work is needed to identify the different types of occupants, employing clustering analysis, then to build the IEQ models for other groups of occupants for various kinds of buildings. Such an analysis could identify the specific requirements or expectations of different types of occupants; based on these models, personalized working/studying environments can be provided. The answers to the ten proposed questions in this study provided an overall picture of the current state of IEQ models and indicated that there are still research gaps in this field. Therefore, future studies are suggested, to be conducted focusing on the following issues:  
* The research methods used to develop IEQ models varied greatly in previous studies and were rarely repeated. A standardized research framework needs to be set up to benchmark IEQ performance on a large scale, including a data measurement protocol, a questionnaire survey, and a data analysis approach targeted at the different data types. The framework should include the core structure and optional branches, which can be customized based on the different investigated building types. In addition to developing the framework, validation tests under other conditions are also necessary for future studies. * Earlier studies discussed the controversial assessment criteria for IEQ performance and indicated a lack of guidance regarding defining the boundaries of IEQ assessment categories. Thus, a set of IEQ assessment criteria with clear definitions of boundary values and the rationales behind the descriptions is demanded. These criteria should be able to be applied to both individual IEQ aspects and the overall IEQ performance. * Considering the differences between occupants, future studies are suggested to develop an IEQ model profile wherein occupants are classified into clusters, based on their IEQ perceptions, and different IEQ models are established for the other occupants.