Related literature

To learn about user demand, user research frequently conducts interviews and questionnaires [20]. It is challenging to uncover every user demand with a questionnaire [21]. Interviews refer to the researchers obtaining the behavioural characteristics and needs of users in the form of question and answer [22]. Interviews are the most widely used and popular data collection method [23]. According to the interview interaction, interviews can be divided into face-to-face, telephone and video interviews [24]. According to the number of interviewees, they can be divided into individual interviews, focus groups, etc [25]. According to the degree of standardization of interviews, interviews can be divided into structured, semi-structured and unstructured interviews [26]. Structured interviews limit the content of questions and the format of answers. The information obtained is simple to process and analyze, but it is difficult to obtain information beyond the questions [27]. Unstructured interviews only set topics and are free to develop around them without the constraints of guides and formats, but are more difficult to conceptualize when used for qualitative analysis in the later stage [28]. The semi-structured interviews set the topic and guide, and the interview guide covers the main topic of the study [29]. It offers a centralized structure for free discussion during the interviews [30]. Semi-structured interviews are a common data collection method in qualitative research and the quality of the interview guide fundamentally influences the results of the study. An interview guide helpes to keep the interviews focused on the topic of the research. The form of a semi-structured guide have a certain degree of flexibility and freedom [31]. The interviewer can adjust the way and order of questions according to the actual situation during the interview [32], so as to obtain the personal feelings and thoughts of the interviewee [33], thereby generating new concepts. Semi-structured interviews typically adopt the traditional face-to-face interactions to accurately capture the participants’ emotional and subtle physical interactions to ask follow-up questions better. With the development of technology, the interviews are no longer limited to face-to-face interaction and have expanded to online communication, including telephone [34] and video chat [35]. For example, Paul suggested using Skype as an interview medium that can overcome access and distance issues and get the closest to the interaction in a face-to-face interview [24].

Oerther divided the semi-structured interview guide questions into warm up questions, core questions, probing questions, wrap up questions [36]. Scott developed a semi-structured interview guide including demographics, event details, Impact from this experience [37]. Young gathered information about interviewees using the APP from 4 directions: intervention characteristics, individual characteristics, inner setting, and outer setting [38]. The consolidated criteria for reporting qualitative research (COREQ) checklist was used to guide design, analysis, and reporting of qualitative results [39]. However, scientific research on semi-structured interviews has focused on the healthcare field, whilst interviews for furniture or product design have no fixed structure. That is, the researcher designs the interview outline by himself. For example, Knauf researched lightweight furniture in terms of general questions (price willing to pay, frequency and location of purchase, demographics), attitudes toward lightweight furniture and materials, and evaluation of materials from multiple perspectives [40].

Grounded theory was proposed by American sociologists Glaser and Strauss in the 1960s [41]. Grounded theory is not an entity theory, but a research path and methodology. Its research aims to generate, develop, and test theories from empirical data, rather than to describe and explain research phenomena [42]. Before conducting grounded theory data analysis, it is necessary to rely on observation and interaction to get relevant textual information about the research problem. It constructs concepts through qualitative coding analysis, induction and synthesis, then generalizes the conceptual categories to obtain theoretical models [43, 44]. It bridges original data and research results, providing a complete set of methods and steps for researchers to generalize and construct theories from raw materials. This theory offers a deeper and more scientific analysis of the influencing factors of customized furniture design. The grounded theory process consists of 3 steps: open coding, axial coding, and selective coding.

Construction of 4E interview-grounded theory model

This paper adopts the semi-structured interview and coding method to construct the 4E interview-grounded theory model in an exploratory way. The 4E interview guide is to conduct semi-structured interviews with interviewees from 4 dimensions: essential information, information extraction, user experience, product expectation.

Essential information is to understand demographics of the participants such as gender, age, and occupation of interviewees [37]. Essential information often appears as the first few questions in the interview or questionnaire so that the interviewer can understand the interviewee’s background. Information extraction refers to the user’s understanding of the product and the way, such as the way to obtain product information, the degree of understanding of the product’s functions. In this era of information explosion, there are various ways for users to obtain information. The ways of accessing product information also differ in the degree of trust in the products [45]. The degree of understanding of product features also influences user demands and product selection to a certain extent [46]. User experience refers to the emotional change of information brought by the user when using and interacting with the product. Norman first proposed the concept of user experience, which has attracted extensive attention from scholars and business circles [47]. It has gradually developed from the initial computer industry to all walks of life. User experience refers to users’ intuitive feelings and feedback when using products, including not only physical and sensory feelings but also emotional and psychological feelings of users in the experience process [48]. Zhu took the government service mobile application as the main body, interviewed the user experience-related issues and formed the evaluation index system of the user experience of the government service mobile application [20]. Product expectation refers to the user’s deeper expectation and demand for the product’s function, material, style and production process. Shiba [49] raised five questions to obtain customer needs: 1)The association when customer using the product. 2)The problems that customer will encounter when using the product 3) The criteria that customer will consider when buying the product. 4) New features that better meet customer expectations. 5) Changes that customer will make to the product. Questions 1 and 2 refer to the user experience. Question 3 refers to the level of understanding of the product in information extraction. Questions 4 refer to the product expectations. Question 5 is not addressed in this study.

The 4E interview guide was able to structure and organize the interview guide of the factors influencing the demand for customized furniture to ensure the integrity of the interview content. The initial coding and initial categories derived from the 4E guide have strong logic and adopted the grounded theory as the analysis method of the 4E interview guide interview text, as shown in Fig 1. The 4E interview-grounded theory model consists of 2 modules: 4E interview and grounded theory coding. 4E interview module is subdivided into 4 dimensions, mainly from 4 aspects: essential information, information extraction, user experience, and product expectations to obtain user information and product demand. The grounded theory coding module mainly processes the collected user information and product requirements. It will conduct data cleaning and coding and finally obtain the constituent factors and mechanisms that affect the user demands for customized furniture.

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Fig 1. Illustration of 4E interview-grounded theory model.

https://doi.org/10.1371/journal.pone.0282956.g001

Research process

Under the guidance of the 4E interview-grounded theory model, the research process of this study is conducted in the following 4 steps, as shown in Fig 2.

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Fig 2. Research process.

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1. At the beginning of the study, the researcher searched relevant papers and analyzed current customized furniture cases to summarize other researchers’ interview questions.
2. Deepen the semi-structured interview questions based on the 4 dimensions in the 4E interview guide.
3. 2 interviewees were selected for the field sample interview test. The interview guide was adjusted and refined according to the interviewees’ responses, as shown in Table 1, which contains main themes and follow-up questions. The main themes grasp the development direction of the interview content, and the follow-up questions are used to gain an in-depth understanding of users’ thoughts and guide users who are too divergent to return to the main themes [50].
4. Interview 36 consumers who have purchased customized furniture or need to purchase customized furniture in recent six months. With the consent of the interviewees, the interview process was recorded. The interview places were the home, studio, or lounge of the interviewees with reasonably control environmental interference. This study has been approved for ethical review by the Science and Technology Office of Nanjing Forestry University. At the same time, all participants signed a written informed consent form before the interview, which included the basic information of this study, the right to voluntary withdrawal and the financial reward.
5. The results of the interviews were coded and analyzed by 2 researchers, and new concepts and categories were collected in a continuous induction process.
6. Saturation tests were conducted on the coding results to construct a model of the influencing factors of customized furniture. In this study, the principle of “theoretical saturation” of grounded theory was followed in the sampling process. In other words, when no new initial concepts, new categories and path relations appeared in the coding process of the original text data of the subsequent interviewees, the primary and secondary categories were determined to be complete and passed the theoretical saturation test.

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Table 1. Customized furniture 4E interview guide.

https://doi.org/10.1371/journal.pone.0282956.t001

Participants

A total of 36 respondents were interviewed for this study, half of whom were male and half female. In order to make the interview results more typical, this study selected people with different occupations and levels of education. The interviewees ranged from 20–55, including 32 young people aged 20–35. This is consistent with the fact that China’s post-90s and post-85s rank first and second in preference for customized furniture in 2020, as mentioned in the Weekly Report of Light Industry and Textile Garment Industry [51].

Open coding

The text description information of the interview materials is coded sentence by sentence in the open coding stage, which is a process of categorizing the interview content [52]. The first 28 interview materials were chosen for coding in this study. As shown in Table 2, 51 concepts and 19 categories were obtained through concept comparison and recombination, including price factors, ease operation, and quality of service.

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Table 2. Partial open coding process.

https://doi.org/10.1371/journal.pone.0282956.t002

Axial coding

In order to determine the internal logical relationships between the inductive categories and the categories created through open coding, the main axial coding is used [53]. As indicated in Table 3, the major axial coding is done based on the results of open coding to create the 4 main categories of fundamental condition, operation behaviour, sensory value, and emotional value.

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Table 3. Partial axial coding process.

https://doi.org/10.1371/journal.pone.0282956.t003

Selective coding

The primary goal of selective coding is to separate the core category from the main categories. The core category of factors influencing user needs of customized furniture is finally extracted through further analysis, comparison, and induction of the 4 main categories obtained from axial coding [53]. The goal of selective coding is to create a story line that describes the relationship between the central category and each main category, as shown in Fig 3.

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Fig 3. Demand factors for customized furniture.

https://doi.org/10.1371/journal.pone.0282956.g003

Theoretical saturation test

Theoretical saturation test refers to the point at which fresh theoretical understandings and test categories can no longer be gleaned from recently gathered data [54]. The study found that no new concepts, categories and relationships were found in the follow-up analysis of the materials of the 29th interviewee. As a result, it is determined that the theoretical research model has reliability and has reached the theoretical saturation state.

Fundamental condition

Through the 3-layer coding based on grounded theory, the sources of fundamental condition can be divided into essential information and information extraction. The fundamental condition obtained from the essential information of user interviews include age, occupation, area, style preference, etc. User essential information plays a vital role in customized furniture design. On the one hand, it is the externalization of the user’s personal needs; on the other hand, it acts as a “filter” of the vast amount of resources and target resources. The fundamental condition derived from information extraction are of a specific social nature. The direct impact of information extraction on customized furniture design is mainly reflected in the social appeal of the internal environment, the display and promotion of the external environment. The social demand of the internal climate mainly refer to the social environment in which the user lives, such as interpersonal relationships, social interaction. The promotion display of the external environment mainly refers to all kinds of media display and platform promotion, which are all kinds of print and video advertisements or promotional information appearing on search engines [55].

The research findings show that: 1) When the recommended information is extracted from strong relationships (friends and family), it will directly stimulate users to experience and purchase offline. 2) When the recommendation information comes from media display or weak relationship recommendation (e.g. forum, twitter), it will stimulate users’ desire for in-depth understanding and indirectly guide users to conduct online and offline brand product queries and other activities. If the information matches the user’s fundamental information, it increases the likelihood that the user will buy. Conversely, when the user’s essential information does not match the enterprise sales positioning, the user chooses to save time costs and does not continue to learn more. 3) When the user’s fundamental condition matches the positioning of the enterprise and products, the possibility of the user’s purchase is greatly increased. Price matching is the most critical factor.

Therefore, the matching degree between the data obtained by users and users’ fundamental condition determines users’ choice of customized furniture. For example, when the product price matches the user’s affordable price, users are most likely to have a benign attitude toward the enterprise and experience the purchase in depth. At the same time, if the information extraction comes from the strong relationship of users, it will greatly increase users’ probability of buying such products.

Operation behaviour

Safe operation, ease operation, product durability and comfortable operation are all included in the category of operation behaviour. All customized furniture operations are predicated on safety, with simplicity of use serving as the fundamental building block, durability of the furniture serving as a guarantee, convenience serving as the core value, and comfort serving as the end aim. Therefore, the operation behaviour is closely related to the user’s functional requirements, which is the consideration of product practicality. User operation behaviour reflects the unique qualities of the user and is highly correlated with usage patterns and prior experiences [56]. That is, the behaviour is generated based on existing cognitive guidance before using the product.

During the interview, more than 90% of the respondents paid close attention to the safety performance of customized furniture. The coded content of the interviews by grounded theory led to the following findings: 1) The influence of the operation behaviour on the demand of customized furniture users is mainly divided into 3 aspects: the user’s physiological condition (height or weight), user habits, and on-site user experience. 2) The on-site user experience is the main factor affecting the user’s choice of customized furniture. For example, when users choose customized kitchen cabinets, they usually choose the kitchen environment of the offline showroom to experience and simulate the cooking process to judge the comfort of the operation. 3) The operation behaviour can directly influence the user’s demand for customized furniture and act as an intermediary factor to trigger the user’s sensory experience to influence the demand.

Sensory value

The furniture contains the connotation of a multi-level sensory system. All the sensory and associative factors related to furniture constitute the complete feeling of a piece of furniture. Vision is the primary source of perception of external things. The visual perception of the product is related to the appearance, color and texture of the customized furniture. Beautiful customized furniture can not only attract users’ attention and bring pleasant feelings, but also stimulate users’ enthusiasm for using furniture. People’s instinctive response to furniture is not just about visual perception, but also the combined appeal of touch, hearing, and smell. When using customized furniture, the material texture and surface finish directly affect the user’s tactile experience. The tactile sensation of the furniture is related to the material texture and surface treatment process. The furniture material is friendly to touch, and this comfortable and pleasant feeling will bring an excellent emotional interaction. The smell or sound emitted by customized furniture is often related to the safety of the furniture panels. For example, the sound brought by the furniture when it is not used smoothly will make consumers think that the product technology is not up to the standard and that there are potential safety hazards. Customized furniture design needs to focus on the integration of multi-sensory to meet users’ living needs and improve user satisfaction. Therefore, the user’s sensory experience determines the user’s demand for the appearance and color, material & finishing of customized furniture.

The findings of the research show that: 1) Based on the ranking choices of the respondents, visual perception is the most influential factor affecting the purchasing power of users in the sensory value. The visual impact often leaves a first impression on users. During the interview process, it was found that the first impression quality can most affect subsequent users’ desire for in-depth experience and purchase demands. 2) The tactile sense is also one of the decisive factors influencing users to purchase customized furniture products. Chinese consumers prefer wooden customized furniture and the warm touch.

Emotional value

Emotional value significantly affects the loyalty, reputation and popularity of customized furniture brands. Based on meeting the fundamental requirements, functional needs and sensory experience of users, it is also important for customized furniture to give emotional value to users. The story behind the customized furniture can arouse the emotional resonance of users. The furniture is used as a medium to convey the brand culture and life philosophy to users. The cultural value and attributes of the product are further transmitted through emotional communication so that users can quickly receive the cultural information and emotional value conveyed by the product. So as to establish a good brand image, create a brand effect, increase user goodwill and viscosity, and improve user purchase probability. Research findings show that brand or product story and brand effect can influence users’ choice of customized furniture. A good brand and product story can give users a deeper understanding of the brand and its products and attract customers’ attention.