Published online July 26, 2022 https://doi.org/10.55396/ined.22.0004
Copyright © Innovation and Education.
Department of Education, Graduate School, Sehan University, Yeongam, Korea
Correspondence to:Nai-Ming Hou
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Due to an unclear definition and a lack of strict quantitative studies, impact of academic inbreeding on research outputs is still controversial. In this paper, the single teacher academic inbreeding index (STAII) was defined to measure the degree of inbreeding by simultaneously considering the educational background and mobility of a teacher. A quantitative study using STAII was carried out to reveal the impact of academic inbreeding on research outputs for 1,715 faculty members from 50 universities or departments of humanities ranked in the top three higher education tiers in China over the last 5 years. Regression analysis indicated positive contributions of academic inbreeding to research outputs with some detailed insights. The STAII was demonstrated to be a reasonable, straightforward and flexible index to quantify academic inbreeding, which would be useful to evaluate and compare academic inbreeding and to provide a reference for talent management in research institutions.
Keywords: Academic inbreeding, Quantitative study, Academic outputs, Humanities, Single teacher academic inbreeding index
The term ‘inbreeding’ comes from biology and refers to the mating of individuals or organisms that are closely related through common ancestry (Kristensen & Sørensen, 2005). The concept of inbreeding in academia was first put forward by Eliot, the former President of Harvard University, in 1908 (Eliot, 1908; McGee, 1960), who named the practice of selecting and hiring new teachers from their own graduates by universities as academic inbreeding. There are many concerns about academic inbreeding (Horta et al., 2011; Smyth & Mishra, 2014). For individuals, inbreeding allows old ideas to continue to prevail without being updated and reformed, resulting in the narrow scope of teachers’ knowledge and the stagnation of professional development. For universities, inbreeding prevents the injection of fresh blood, so it leads to narrowness and affects the academic openness of universities and is not conducive to the improvement of scientific research creativity. Inbreeding will also cause the irrationality of human relationship, academic corruption, and affect the democracy of university management. Although Chinese traditional culture has always emphasized academic inheritance, the adverse effects of inbreeding have been more and more recognized by government agencies and academia. Recently, the national medium and long term plan for the development of educational talents (Xia, 2014) proposes to improve the learning relationship of teachers in colleges and universities and to gradually reduce and eliminate the phenomenon of academic inbreeding by encouraging colleges and universities to significantly reduce or not select their own graduates in teacher recruitment. The impact of academic inbreeding on the academic ecosystem is so extensive that educational quality and the professional development of individual teachers (Omingo, 2019), as well as the innovation capability of academic institutions and even of all of academia, will be more or less affected by it (Tavares et al., 2015). Therefore, the study of academic inbreeding in colleges and universities is of great theoretical significance and practical value.
At present, academic inbreeding is generally used to refer to the phenomenon of colleges and universities retaining their own graduates as teachers (Eells & Cleveland, 1935a; Horta et al., 2011; Smyth & Mishra, 2014). Nevertheless, the strategies used to define the academic inbreeding individuals or groups are inconsistent, and even ambiguous views exist about the meaning of the term. Generally, there are two strategies to define academic inbreeding: educational background and academic mobility. The strategy of educational background is to consider the educational relationship between teachers and their universities of employment, and the situation of teachers with the same educational background of the university where they work is defined as inbreeding. According to different educational levels, the educational background can be divided into the undergraduate, master’s and doctoral levels. Therefore, there is no unified standard for the definition of inbreeding by educational background. Some researchers have adopted a relatively broad definition and believe that as long as teachers have obtained a degree in their institutions of employment, they are regarded as engaging in inbreeding regardless of the level of academic degree (Eells & Cleveland, 1935a). Other scholars narrowed the scope and adopted a more specific definition (Berelson, 1960; Austin & McDaniels, 2006). In the early days, scholars who studied American missionary universities regarded the employment of bachelor’s degree graduates of the university as inbreeding (Clark & Larson, 1972). With the doctoral degree becoming a necessary condition for the appointment of teachers in colleges and universities, European and U.S. higher education institutions began to pay attention to the educational background at the doctoral level, clearly defining inbreeding as the employment of graduates of doctoral programs of the university, regardless of whether their bachelor’s or master’s degree was also obtained from the same university (Eisenberg & Wells, 2000; Horta et al., 2010; Inanc & Tuncer, 2011). In Asia, especially in China, the phenomenon of inbreeding at the undergraduate level has received ongoing attention (Altbach et al., 2015). In addition, some researchers have suggested a stricter definition of inbreeding, claiming that only hiring teachers with the first and highest academic degrees from universities of employment is inbreeding (Sanz- Menéndez & Cruz-Castro, 2019). Some authors even advocate the most stringent definition of inbreeding as having obtained all 3 degrees (bachelor’s, master’s and doctoral degrees) from the school of employment (Shen, 2016).
When inbreeding is defined by mobility, it is associated with teachers’ nonmobility or low mobility and is regarded as the opposite of teachers’ academic mobility (Horta & Yudkevich, 2016). Some studies have focused on the working experience of university teachers after their doctoral degrees and believe that they can be considered to engage in academic inbreeding only when they are directly employed by their universities after obtaining their doctoral degree and have no working experience in other institutions (Berelson, 1960). Other studies distinguish two phenomena in teachers’ working experience after finishing the degree. One is the case of further studies and working in one’s alma mater after undergraduate graduation, which is called ‘pure inbreeding’ (Dutton, 1980; Horta, 2013); the other case is to return to one’s undergraduate alma mater to start a career after completing graduate education (or even postdoctoral training) in other colleges and universities, which is called ‘mobile inbreeding’ (Horta, 2013). In addition, the concept of ‘the silver-corded’ was first put forward in 1958, referring to teachers who first accumulate working and research experience in other universities after graduation and then return to their doctoral alma mater to obtain a teaching position (Caplow & McGee, 1958). There are disputes about the identification of the ‘silver-corded’. Some studies classify them as a kind of academic inbreeding (Lawson & Shibayama, 2015), but some studies claim that they have a high level of mobility, given that their return depends more on their own qualifications rather than special interest by their alma mater, these cases should not be considered as inbreeding (Horta, 2013).
A common concern among researchers and administrators about inbreeding is its adverse impact on academic productivity, but the existing empirical research results have not reached consistent conclusions. There are mainly three different views: (1) Inbreeding has a negative impact on teachers’ academic productivity. In 1908, Eliot believed that inbreeding was dangerous to the development of universities, which was the earliest comment on academic inbreeding (Eliot, 1908). A study in 1935 found that noninbred teachers performed significantly better than inbred teachers in promotions, academic output and professional recognition (Eells & Cleveland, 1935b). Peltz & Andrews (1996a) explained that academic inbreeding was inferior to the lack of inbreeding from the perspective of teachers’ academic innovation ability. Darden further clarified the view that inbreeding, as a static and immobile state, has a negative impact on teachers’ academic development by comparing the influence of inbreeding and occupational mobility on scientific research output (Dutton, 1980). The negative impact of inbreeding on academic output has been also supported by a series of other studies (Hollingshead, 1938; Hargens & Farr, 1973; Pelz & Andrews, 1996b; Eisenberg & Wells, 2000; Soler, 2001; Horta et al., 2010; Horta, 2013). In China, some researchers have shown that noninbred teachers have certain advantages in some aspects of research output (Xia, 2014; Zhang & Shen, 2015); (2) Inbreeding has a positive impact on teachers’ academic productivity. The first research study finding a positive impact of inbreeding on academic output comes from a survey of the American academic labor market by Caplow & McGee (1958). In 1960, McGee, based on the research from the University of Texas, concluded that academically inbred faculty outperform noninbred faculty in terms of the number of papers and studies published (McGee, 1960). In 1984, Wyer & Conrad studied 160 universities in the United States, and the sample size increased to 3,054. They concluded that although inbred teachers dedicate less time to teaching and research, they are more productive than noninbred teachers in all aspects (Wyer & Conrad, 1984). Since the 21st century, some researchers continue to adhere to the view that inbreeding has a positive impact and provide new evidence (Tavares et al., 2015). Some Chinese scholars also advocate for the positive effects of academic inbreeding, such as Yan (2008); (3) Inbreeding has no significant influence on teachers’ academic productivity, which is the position of authors including Cruz-Castro & Sanz-Menéndez (2010), Long (1978), and Smyth & Mishra (2014). Studies in China with similar views include Zhang & Shen (2015).
There are an increasing number of definitional, speculative and empirical studies on academic inbreeding, which shows the importance of this topic. However, there are still some difficulties and deficiencies in the current empirical and quantitative research; that is, the definition of the scope and degree of academic inbreeding is not clear, and there is a lack of accurate and effective quantitative methods to measure academic inbreeding, helping explain why different quantitative studies are incomparable or even reach different conclusions. For example, when dealing with educational background, which academic degree obtained from the school of employment should be recognized as inbreeding? Both strategies tend to deal with academic inbreeding as static. With the diversification of teachers’ professional development paths and the high frequency of teachers’ professional mobility, the traditional binary classification of inbred and noninbred has become inadequate because there is a transition interval between the two conditions, and they are usually interchangeable. For example, when the educational background approach is used to define academic inbreeding, teachers initially identified as engaging in academic inbreeding may later be classified as noninbred or less inbred due to changes in affiliations. Similarly, when the mobility approach is used to define academic inbreeding, the same faculty member’s identity may change from one of pure academic inbreeding to the silver-corded and to noninbred at different stages in their career. Obviously, the educational background strategy and mobility strategy adopted in the current research pay little attention to the dynamic changes in teachers’ professional development. Therefore, the existing definition is insufficient in the evaluation and comparison of academic inbreeding, as well as in analyzing its impact. A reasonable way to tackle the above problems is thus to return to the essential conception of academic inbreeding by unifying educational background and mobility.
To evaluate and compare academic inbreeding, this paper attempts to unify educational background and mobility in a more flexible index called the single teacher academic inbreeding index (STAII). Based on the STAII and average STAII, the relationship was studied between the degree of academic inbreeding and research outputs of 1,715 teachers in 50 humanities schools or departments from the top three tiers in China in the last five years.
When defining and quantifying the academic inbreeding relationship between a single teacher and the institution of employment, this paper considers educational background and mobility at the same time and is based on the following considerations: (1) Educational background, including obtaining undergraduate, master’s and doctoral degrees in the school of employment, should always make positive and cumulative contributions to academic inbreeding; (2) Mobility, according to the years of substantial study and research outside the university of employment, should always make negative and cumulative contributions to academic inbreeding; (3) Once a teacher obtains one or more academic degrees in the university of employment, the academic influence of the educational background on the teacher will always exist; namely, its contribution to academic inbreeding will not be completely offset by mobility, regardless of the number of years; and (4) The value of the STAII index is between 0 and 1, where 0 represents no academic inbreeding, and 1 represents the strongest academic inbreeding, that is, pure inbreeding. Based on the above considerations, this paper defines the STAII as follows:
where B, M, and D represent the scores of the bachelor’s, master’s and doctoral degrees obtained in the school of employment, respectively, which can be set according to the specific conditions of the educational environment. Since the research object of this paper is Chinese universities, by referring to the general number of years of study to obtain academic degrees in China and the results of academic inbreeding research in China, they are assigned values of 0.2, 0.3, and 0.5, respectively. The last item on the right in Equation (1) is the contribution of academic mobility. Y is the number of years of mobility, which refers to the number of years of substantial study and research in institutions other than the school of employment. YD is the general number of years of study required to obtain a doctoral degree. In this paper, YD is taken as 4 years according to the general situation of Chinese universities. The second item in the bracket shows that study elsewhere will increase mobility, but regardless of the years of mobility, the influence of educational background (obtaining academic degrees in the university of employment) cannot be completely eliminated, and the contribution of educational background is offset by half at most. Obviously, the range of STAII is between 0 and 1. When bachelor’s, master’s and doctoral degrees are obtained in the same institution where the faculty member works, without any mobility, this can be considered ‘pure inbreeding’ (Dutton, 1980; Horta, 2013), for which the STAII obtains the maximum value of 1. When a faculty member obtained the bachelor’s, master’s, or doctoral degrees at institutions other than where they are employed, the STAII will have the minimum value of 0. In addition, the STAII has many continuous values to represent other statuses.
The original data used in this study, including age, gender, professional title, educational background, mobility and the number of papers published in the past five years (from June 30, 2016 to June 30, 2021), were collected for 1,736 teachers from 50 universities (departments) of humanities ranked in the top three tiers in China, including level A++, level A+, and level A, from the highest to lower levels. Among them, age, gender, professional title, educational background, and mobility are obtained from the school website and other public sources. The published papers in Chinese were retrieved from the China national knowledge infrastructure (CNKI) database, and the English papers published were searched in the Scopus database.
The dependent variable of this study is the productivity of teachers’ research achievements, which are measured by the number of published papers. Considering the channels of academic publication, academic output is further divided into three classes: English papers (Social Sciences Citation Index, SSCI) and domestic publications (Chinese Social Sciences Citation Index [CSSCI] and non-CSSCI). Considering the primary and substantial contribution of academic publications, only those papers in which the teacher is first author or corresponding author are included. The former reflects the overall academic ability of a teacher, and the latter reflects the academic ability as a research leader (Roberts & Seaman, 2018). The independent variable is the degree of academic inbreeding. This paper adopts the newly proposed STAII to measure the degree of academic inbreeding for a single teacher, and the average value of the STAII can be used for a group of teachers.
To quantitatively study the relationship between STAII and academic output, the influence of other factors should be controlled and reduced as much as possible. In this paper, incomplete and unbalanced randomized block design (IURBD) (Gupta et al., 1991) is adopted in the random survey of teachers, and the following variables are established as blocking factors: (1) gender of a teacher, divided into woman and man; (2) age of a teacher, divided into four groups: 36–40 years old, 41–45 years old, 46–50 years old, and 51–55 years old; (3) Discipline level and university reputation, which are divided into three levels: A++ (1st), A+ (2nd), and A (3rd); and (4) academic title, divided into three groups: lecturer, associate professor, and professor. Generally, IURBD makes the distribution of sampling and surveys at all levels of blocking factors as random as possible. In addition, to eliminate the influence of academic degrees, the research object is limited to university teachers who have obtained doctoral degrees.
The data analysis of this study was carried out on the MATLAB 7.0.1 platform (MathWorks, Sherborn, MA, USA) using self-compiled MATLAB codes. The relevant data and the code for calculating STAII can be requested from the corresponding author. A
In the survey, the individuals with missing data, such as educational background, professional career and paper output, were eliminated, and the final number of effective objects was 1,715, including 1,221 men and 494 women. According to the level and reputation of disciplines, the sample was divided into 16 level A++ universities, 18 level A+ universities, and 16 level A universities, including 570, 639, and 506 teachers, respectively. The age group included 425 persons aged 36–40 years, 409 persons aged 41–45 years, 437 persons aged 46–50 years, and 444 persons aged 51–55 years. In terms of professional titles, there were 309 lecturers, 604 associate professors, and 802 professors. The distribution of the above individuals generally meets the sampling requirements of IURBD.
The STAII is used to quantitatively evaluate the academic affinity relationship between a single teacher and the school of employment. In Equation (1), the mobility years of a teacher refer to the sum of years engaged in postdoctoral and visiting scholar experiences and other activities involving substantive learning and research outside the school of employment. Here, work experience unrelated to substantial learning and academic research is excluded, e.g., work as a middle school teacher after obtaining a bachelor’s or master’s degree. The following is an example of how the STAII is calculated. Suppose that a teacher has obtained both undergraduate and master’s degrees in the university of employment, obtained his or her doctoral degree in another university, and has a total mobility period of 4 years, according to Equation (1), B = 0.2, M = 0.3, D = 0, Y = 4, and YD = 4 (the general period of obtaining a doctoral degree in China is 4 years). The calculation of STAII is as follows:
The individual’s bachelor’s and master’s degrees make a positive contribution of 0.5 to academic inbreeding. Having been engaged in study and research elsewhere for 4 years (equivalent to the average time to obtain a doctoral degree in China) provides a contribution of —0.25 to academic inbreeding in terms of mobility. In conclusion, the STAII is 0.25, which seems to be a reasonable and proper value.
In our definition of STAII, the contributions of educational background and mobility to academic inbreeding are cumulative, and they can offset each other. When defining whether a teacher has engaged in academic inbreeding, different STAII thresholds can be adopted according to different research purposes and emphases. In this paper, the author considers STAII > 0; that is, as long as a teacher has obtained a higher education or advanced degree in the school of employment, he or she will be regarded as engaging in the practice of academic inbreeding. Among the 1,715 teachers, 937 teachers (STAII > 0; 54.64%) were considered to participate in academic inbreeding, and 778 teachers (STAII = 0; 45.36%) did not engage in inbreeding. The average STAII of all 1,715 teachers was 0.283. Since all teachers are randomly selected, the data also reflect the average degree of academic inbreeding in the three levels of universities.
In addition, the proportion of academic inbreeding at different levels of each blocking factor was also studied, as shown in Table 1. According to teachers’ gender, 59.38% (725/1221) of men teachers were more or less inbred, while the proportion of women teachers with academic inbreeding was lower, at 42.91% (212/494). According to age, the proportion of academic inbreeding of older teachers was higher (77.57% for the 46–50 years group and 56.08% for the 51–55 years group). This difference is very interesting because it shows that the degree of academic inbreeding among college teachers has changed over the years. In addition, the average STAII values of the 4 groups, that is, 51–55-years-old, 46–50- years-old, 41–45-years-old, and 36–40-years-old, were calculated as 0.297, 0.392, 0.208, and 0.228, respectively. The average STAII reached the maximum value of 0.392 among teachers aged 46–50 years, which could be attributed the fact that in a certain period of time, Chinese universities tend to employ their own graduates as teachers, because doctors returning from abroad are rare and the scale of doctoral education in China is relatively small. Interestingly, among the younger teachers aged 41–45 years and 36–40 years, the overall academic inbreeding degree is decreasing, indicating that in these universities young teachers have more diversified educational backgrounds and greater mobility. In the youngest group investigated, namely, 36–40-years-old, the average STAII was at a low level of 0.228. The above trend is related to the fact that top universities in China have recently recruited faculty with more overseas experience and mobility. It also shows that universities are increasingly interested in avoiding excessive academic inbreeding when hiring teachers.
Table 1 . Distribution of teachers with different degrees of academic inbreeding (STAII > 0) at different factor levels.
|Factor||Percents of inbreeding|
|Title (inbred vs. noninbred)|
|Lecturer||17.50 vs. 18.64|
|Associate professor||30.10 vs. 41.39|
|Professor||52.40 vs. 39.97|
STAII = single teacher academic inbreeding index..
In addition, the STAII was used to compare academic inbreeding at different factor levels. The number of teachers from level A++, level A+, and level A with different degrees of academic inbreeding (STAII > 0) reached 75.96% (433/570), 35.99% (230/639), and 54.15% (274/506), respectively, and the average STAII values were 0.376, 0.213, and 0.267, respectively. Level A++ institutions are the top-ranked universities, and it is not surprising that their academic inbreeding proportions are the highest, as they tend to recruit excellent graduates from the best schools, including many of their own graduates. This can help to maintain their excellent academic tradition and ranking, which is consistent with the results of previous research (McGee, 1960; Wyer & Conrad, 1984). With a high academic reputation and running resources, Level A+ universities also tend to, and have the opportunity to, recruit more graduates from level A++ universities and overseas schools, which helps account for the fact that their proportion of academic inbreeding is the lowest. Compared with level A+, due to relative weakness in academic reputation and resources, level A universities recruit a higher proportion of their own graduates, so their proportion of academic inbreeding is in the middle.
To investigate the impact of academic inbreeding on professional promotion, this paper compares the distribution of academically inbred (STAII > 0) and noninbred teachers (STAII = 0) at different professional title levels, as shown in Fig. 1. The proportion of lecturers among inbred teachers and noninbred teachers is nearly the same (17.50% vs. 18.64%), as PhDs in Chinese universities can generally obtain lecturer titles, and academic inbreeding has almost no impact on obtaining lecturer titles. The proportion of inbred faculty and noninbred faculty among associate professors was 30.10% vs. 41.39%, respectively, and the proportion of inbred faculty and noninbred faculty among professors was 52.40% vs. 39.97%, respectively. Considering these two comparisons shows that academically inbred teachers are more likely to obtain professor positions. This is consistent with previous research results (McGee, 1960; Wyer & Conrad, 1984). It is believed that academic inbreeding is not only conducive to maintaining good academic traditions and disciplinary advantages but also conducive to teachers’ adaptation to the environment and the use of academic and social resources to pursue professorships.
To quantitatively study the relationship between academic inbreeding and research output, this paper weights and scores research output. The scores of each paper are established as follows: ordinary Chinese papers, 2 point; CSSCI paper, 5 points; and SSCI paper, 10 points. First, we divided all 1,715 teachers into two groups: inbred teachers (STAII > 0,
Table 2 . Comparison of output scores of inbreeding and noninbreeding under different factor levels.
To further study the quantitative relationship between the degree of academic inbreeding and research output, we take the average scientific research output of noninbred teachers in the same college as the control and perform a regression analysis on the output score (OS) of each inbred teacher (STAII > 0, n = 937). The regression analysis plot is shown in Fig. 2. The purpose of this analysis is to reveal the correlation between the above two variables. Therefore, no treatment is made on the possible regression outliers. The obtained regression equation is as follows:
As shown in Fig. 3 and Equation (2), although univariate regression can only explain 25.8% of the data variance, the regression equation is statistically significant (
Finally, we use the average STAII as an index to measure the degree of academic inbreeding universities. At each discipline level, we performed a regression analysis of the average scientific research output score (AOS) of each school to its average STAII. The regression analysis diagram is shown in Fig. 3. The purpose of this analysis is to reveal the correlation between the above two variables. Therefore, we do not exclude possible regression outliers. The regression equations of universities and universities at three levels are as follows:
For A++ universities, academic inbreeding has a statistically significant positive contribution to the average academic output, which can be attributed to good academic genes and academic traditions because A++ universities and universities tend to give a considerable number of teaching posts to their own graduates. The situation of A+ universities is slightly different. Although the contribution of STAII to the average research output is positive from Equation (4), the regression only explains 16.2% of the data variance, which is not statistically significant. As mentioned above, while A+ universities retain their graduates, they can also recruit a considerable number of A++ and students with overseas experience because of their good status and resources. This reduces academic inbreeding to some extent, and some noninbred individuals also provide higher academic output. For level A universities, academic inbreeding has a statistically significant negative contribution to average academic output, which is due to the high academic inbreeding rate caused by the large number of graduates retained in these universities. On the other hand, the graduates recruited by level A from more renowned academic institutions also make a considerable contribution to scientific research output. This shows that, at least for level A universities, a higher academic inbreeding rate does not improve academic output and may even be harmful to academic output.
In view of the unclear definition of academic inbreeding, the lack of strict quantitative studies and consistent understandings of the impact of academic inbreeding, this paper revisited the definition of academic inbreeding and proposed a new index to measure academic inbreeding. Based on this index, we perform an empirical study on the impact of inbreeding on academic output in 50 humanities colleges/departments in China.
First of all, STAII and ASTAII proposed in this paper are flexible and reasonable indicators for evaluating and comparing academic inbreeding, both for individual teachers and teacher groups. They can be used for quantitative research of academic inbreeding and provide reference for talent management of scientific research institutions. STAII and ASTAII have the advantage of integrating academic relationship and mobility. Because they cannot distinguish the above two factors, extra parameters may be needed in special research of academic relationship and mobility. However, in the definition of STAII proposed in this paper, the idea that each of them has an accumulative contribution may still be useful for measuring the academic relationship and mobility separately.
Secondly, the empirical study on the impact of inbreeding on academic output of the first three levels of humanities in Chinese universities shows that we cannot draw an arbitrary conclusion. When academic inbreeding is controlled in a reasonable range, it helps to maintain excellent academic genes, allocate academic resources reasonably, communicate and cooperate among researchers, so inbreeding can promote research output; On the other hand, when academic inbreeding is excessive and the above advantages cannot be reflected, it is wise to recruit high-level teachers from outside. In addition, academic output depends on many factors, such as age, gender, professional title, motivation, environment, policy and so on. Therefore, when STAII and ASTAII are used to measure inbreeding, the level of other factors should be strictly controlled. The practicality of STAII and ASTAII needs more empirical studies to prove.
Finally, in this study, the critical value of STAII is set as 0 when distinguishing inbreeding and non-inbreeding. Although this seems reasonable, there are still other possible choices. Another reasonable consideration might be to divide the degree of inbreeding into multiple levels according to the value of STAII by reference to the values of some selected groups. It may be conducive to more accurate empirical research, which will be further investigated in our future work.
Raw data are available upon request from the corresponding author.
The authors declare that they have no competing interests.
No funding was received for this study.
Nai-Ming Hou was responsible for conceptualization, methodology, literature review, resources, writing original draft, and data analysis.
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