Abstract
Attempts to meet the increasing demand for long-term care (LTC) services have been hindered by acute staff shortages and high turnover. Distinct from previous studies, a descriptive phenomenological approach with van Kaam’s controlled explication method was adopted in this study, to delineate how attributes of the LTC work environment shape the workforce crisis. Individual interviews were conducted with 40 LTC workers from 10 facilities in Hong Kong. The results suggest that the work environment in LTC facilities is not only characterized by organization- and job-related attributes that influence staff outcomes but also is a socially constructed concept with derogatory connotations that can influence staff recruitment and retention. Concerted efforts from facility administrators and policy makers are needed to improve the quality of the work environment. Future initiatives should focus on developing a vision and strategic plan to facilitate the rise of the LTC sector as a profession.
Introduction
An aging population and the resulting increasing demand for long-term care (LTC) services is a global concern (World Health Organization, 2015). The need to expand the capacity of LTC services is clear; yet, the phenomenon of workforce shortages has posed critical challenges to such an endeavor (Colombo, Llena-Nozal, Mercier, & Tjadens, 2011). Hong Kong, a well-developed region with a Western health care system, is no exception. LTC facilities that provide both residential and nursing care for the older adults are at the heart of the workforce crisis. The vast majority of these facilities, locally known as “care and attention homes,” are intended to serve older adults with moderate levels of impairment, whereas a small number of “nursing homes” exist to care for the more severely impaired. They provide a mixture of government-subsidized and nonsubsidized LTC places. The core workers in the LTC sector include personal care workers (PCWs), who look after residents’ activities of daily living; certified nursing assistants (CNAs), who oversee PCWs’ work and are responsible for the delivery of basic nursing care; and nurses who provide nursing care and monitor the quality of that care. Shortages of LTC workers at all ranks in the various types of LTC facilities are common and have become a critical public concern (Hong Kong Council of Social Service, 2015; Legislative Council Secretariat, 2016). Much effort has been devoted to improving the supply and retention of LTC workers, primarily by providing training opportunities to attract new entrants and funding support for LTC facilities to improve staff salaries and recruit more workers (Legislative Council Secretariat, 2013). Yet, the long-standing problem of acute staff shortages and high turnover persists (Audit Commission, 2014).
There has been a call for the need to improve the quality of the work environment. A wide range of work environment attributes that have a significant bearing on LTC staff satisfaction and turnover have been identified in survey-based studies. These comprise workload and training (Castle, Engberg, Anderson, & Men, 2007); leadership styles (Chu, Wodchis, & McGilton, 2014), coworker support (Gao, Newcombe, Tilse, Wilson, & Tuckett, 2014), staff autonomy (Culp, Ramey, & Karlman, 2008), role expectations (Rai, 2013), and work relationships with families (Abrahamson, Suitor, & Pillemer, 2009). From a positivist stance, these studies have focused mainly on predetermined organization- and job-related characteristics that influence staff outcomes. Some scholars have noted an overemphasis on the use of instruments to measure job satisfaction and have attempted to shed greater light on the issue of the LTC workforce by using qualitative methods (Moyle, Skinner, Rowe, & Gork, 2003). The qualitative methods have uncovered less quantifiable and implicit attributes underlying the issue, such as religious beliefs or spirituality (Mittal, Rosen, & Leana, 2009), the meaning of work (Secrest, Iorio, & Martz, 2005), and the importance of the cultural backgrounds of the workers (Gao, Tilse, Wilson, Tuckett, & Newcombe, 2015). This suggests that “methodological diversity” can contribute to generating new insights on a complex phenomenon (Schwandt, 2012). Thus, in an attempt to better understand how the LTC work environment shapes the workforce issue, a descriptive phenomenological approach was adopted in this study.
Design and Method
Phenomenologists believe that by generating rich descriptions of, and identifying the commonalities in, the lived experiences of individuals, the essence of a phenomenon can be uncovered (Lopez & Willis, 2004). In this study, attributes of the work environment emerged as the essence in shaping staff outcomes, contributing to the phenomenon of workforce shortages and high staff turnover. By elucidating the lived experiences of LTC staff as they worked in the LTC facilities, the essential elements constituting the phenomenon were captured. The processes of data collection and analysis were further guided by van Kaam’s (1966) systematic method of controlled explication, which is composed of two phases: the “prescientific” and “scientific.”
The “Prescientific” Phase of Controlled Explication
The “prescientific” phase involves a process of translating the lived experiences of individuals into more concrete knowledge of the issue under study (van Kaam, 1966). In this study, information on the lived experiences of LTC workers was elicited through individual, face-to-face, and semistructured interviews that were conducted using an interview guide (Table 1). The informants were asked to share their experiences of working in LTC facilities, and questioned on how these experiences shaped staff outcomes in terms of their sense of satisfaction with their job and their turnover intention. The inquiry focused not only on descriptions of what were experienced but also on why such experiences occurred, and how the experiences related to the social context and phenomenon under study.
Interview Guide.
Note. LTC = long-term care.
To ensure that informants have some insights on their work and work environment, only those with at least one year of work experience in the LTC sector were sampled. Snowball and maximum variation sampling were adopted to recruit individuals with as wide a range of characteristics as possible (e.g., rank and years of work experience), so as to maximize the experiences being captured. A total of 40 LTC staff (Table 2) from 10 facilities were involved, including 15 nurses, 12 CNAs, nine PCWs, and four administrators from a variety of publicly and privately funded facilities. All the interviews were conducted from March 2015 to April 2016; all lasted from 60 to 90 minutes, and were audio recorded and transcribed verbatim for analysis. Ethical approval to conduct the study had previously been obtained from The Open University of Hong Kong. The written consent of the informants to participate in the study was then sought and obtained.
Demographic Characteristics of the Informants.
Note. LTC = long-term care.
This group was composed of seven registered nurses and eight enrolled nurses (equivalent to “licensed practical nurses” or “licensed vocational nurses”), and they shared similar work responsibilities in the sampled LTC facilities.
The “Scientific” Phase of Controlled Explication
The “scientific” phase involves a process of explicating the textural descriptions and validating the meanings of the lived experiences to achieve a synthesized account of the phenomenon (van Kaam, 1966). This can be achieved through the following six steps: listing and preliminary grouping, reduction, elimination, hypothetical identification, application, and final identification (van Kaam, 1966). During the process, the researchers read and reread the interviews to gain an intuitive understanding of the unique experiences of each informant. Their narrative descriptions were listed and compiled in word processor files, and were jointly reviewed by researchers to identify the initial categories for grouping the essential elements relevant to the experiences and phenomenon under study. The attributes of the work environment emerged as the essential elements, which were then interpreted through a series of systematic processes. These involved revisiting the clustering of the essential elements, and the meanings and relevance of the descriptions, reducing and eliminating overlapping and extraneous descriptions, and modifying vague descriptions into more precise terms. To validate the findings of the study, a hypothesis was generated based on the extant literature (referred to by van Kaam as an “external judge”). It was postulated that the attributes of the work environment shape staff outcomes. LTC workers who regard the attributes of the work environment as positive would report a sense of job satisfaction and an intention to stay in the LTC facilities, and vice versa. As for the processes of application and final identification, the hypothesis was “applied” by clustering the informants according to their perceptions of job satisfaction (satisfied and dissatisfied) and turnover intention (intended and did not intend to leave). Their narrative descriptions were further examined to verify the attributes of the work environment as identified from the study.
Trustworthiness of the Study
A number of quality-enhancement strategies were incorporated to ensure the trustworthiness of the findings, namely, bracketing and data and investigator triangulation. During the research process, the researchers continuously reflected on the presence of any personal biases or prejudices that might have affected the course of inquiry and analysis. The findings of the study were generated from multiple sources, including informants at different ranks, with varying years of work experience and from different types of LTC facilities. Their lived experiences of working in the LTC facilities were compared to validate the emerging findings. Researcher biases were further minimized by having three members of the research team involved in collecting and analyzing the data to ensure that the findings were grounded in the data and adequately captured the reality of the phenomenon.
Results
Informants described in the interviews that they were working in “a highly complicated and challenging work environment,” and indicated that the resulting job dissatisfaction and turnover intention were counteracted by the presence of several “support systems.” Further analysis of informants’ lived experiences of working in LTC facilities contributed to the development of a conceptual framework (Figure 1) that captures attributes of the work environment influencing staff outcomes. How individuals weigh the positive and negative aspects of their work environment determines their decision on whether to stay or leave a job. Those who provided more illustrations associated with the negative attributes of the work environment tended to report unfavorable job outcomes, whereas those who perceived more support at work tended to be satisfied and willing to stay. The main findings of the study are delineated below, annotated with extracts from the transcripts.
A conceptual framework of the attributes of the work environment in LTC facilities for the elderly.
A Highly Complicated and Challenging Work Environment
Informants described working in a complicated and challenging work environment. The following emerged as the most crucial aspects of that environment, ranging from the micro job-related to macro societal-related attributes: high job demands, compromised team effectiveness, organizational limitations, and the devaluation and demoralization of the workforce.
High job demands
The informants identified four critical issues leading to high job demands. First, managing difficult family members emerged as the main issue of concern among the informants. They described family members as “vexatious” or “too demanding,” and attributed this issue to three factors. The first is the rise of a culture of consumer rights and complaints, which a nurse described as follows: “They (the family members) just feel like if they don’t speak out, they won’t get what they want” (Nurse). The second is the problem of misunderstanding and distrust, which causes family members to question and complain about the quality of care. Finally, the informants recognized that family members were being difficult because “They are used to looking after their parents one-on-one at home . . . and feel guilty about having dumped their parents in our place” (Nurse).
Second, the informants reported that it was difficult to look after residents with various health needs. The need to provide highly complex care stems from three factors. First, under the existing framework regulating the admission of older adults to an LTC facility, residents with or without mental illnesses (e.g., anxiety and bipolar disorders) are permitted to live in the same facility. Second, residents in care and attention homes who need a high level of care could potentially be transferred to nursing homes, but this rarely happens because of the lack of such places: “Some residents may have passed away while waiting for a Nursing Home placement” (Nurse). Finally, care and attention homes that are intended to serve moderately impaired residents are motivated by government financial incentives (e.g., additional public funding such as the “infirmary care supplement”) to accept and retain those residents who require a higher level of care.
Third, the need to handle unpredictable incidents at work is another source of stress. Unpredictable incidents have included a sudden deterioration in residents’ health condition, resident-to-resident aggression (e.g., fighting), and resident behavioral disturbances (e.g., running away). Other than health issues, LTC staff have to cope with incidents for which they are unprepared, such as a sudden breakdown in the facilities: “A water pipe burst in a bedroom . . . We were never taught how to manage this, and yet you have to deal with it here” (Nurse).
Fourth, the workforce issue inevitably led to LTC workers at all ranks being forced to take on duties not in alignment with their work roles. In facilities where psychosocial services were available for residents and yet were understaffed, nurses were asked to play the role of a social worker to develop social activity plans for residents. Many CNAs reported having to do the work of PCWs, whereas PCWs also cited having to take on cleaning and other housekeeping duties when these jobs were understaffed, resulting in an increased workload and work stress.
Compromised team effectiveness
In response to workforce shortages, many LTC facilities turned to health care employment agencies and/or hired older workers. Informants were aware of the existence of two distinct work relations in their work environment. First, it was intergenerational incompatibility among coworkers, which was rooted in a “knowledge gap” between two generations who had received different training. Confusion over “best practices” was often reported: “They (older nurses) have been practicing what they learned years ago . . . We have different ways of doing things, so it’s a bit confusing now” (Nurse). Lack of compliance also stemmed from seniority, in terms of being older or having more years of work experience: “PCWs are older, they have their own views about what’s right and wrong, and they just go their own way” (CNA).
Second, the informants recognized that agency workers were less efficient and independent primarily because they were unfamiliar with the facilities and residents. Nurses complained that the need to supervise agency workers created extra work for staff: “We need to take care of our own routine, and also to take the time to help them (agency workers) get things done” (Nurse).
Organizational limitations
It was noted that the structural or functional constraints in an organization hampered work effectiveness and led to increased workloads and/or stress. With regard to physical settings, limited spacing between beds impeded the staff from working in pairs and using lifting devices. In addition, all the sampled LTC facilities were located in multistory buildings, and some buildings did not have elevators. This resulted in more labor-intensive work for staff, and increased attrition and/or injuries at work: “I had to go up and down the stairs all the time. My claves always cramped up when I was asleep” (CNA).
Some nurse informants compared their present job with their previous work experience in hospital settings, where internal support structures such as those for managing facilities and risks are well established. Those structures were largely absent in the LTC facilities, and staff found themselves having to cope with complex issues on their own: “No one is there to save you from an incident, like ringing fire alarm, or runaway resident” (Nurse).
Devaluation and demoralization of the workforce
Apart from the difficult work environment at the facility level as described above, findings also revealed that the informants viewed their work environment as comprised of more than organization- and job-related elements. The quality of their work environment was closely linked to two societal factors, namely, “social stigmatization” and the “lack of professionalism” of the sector, resulting in a socially devalued and demoralized work environment, which affects staff recruitment and retention.
Social stigmatization of the work environment was embedded in the LTC workers’ internalization of social discourses about their job and workplace. Many informants cited negative responses from their relatives or friends when they told them that they worked in an LTC facility: “Oh, you work for an elderly home! Your body has an elderly smell!” “What do you work there for? To change diapers?” Stigmatizing labels such as “dirty work,” “pee and poop,” “awful smell,” and the like were commonly reported. The informants attributed this to the way LTC facilities are portrayed among the public and in the media: “Many people say that this is a horrible place, a place that is horribly smelly, and where the older adults get abused and wait for death” (Administrator). Stigmatization discourages people from entering the sector, and some informants called for the construction of a more positive image for LTCs: “Society as a whole treats our work as low-status . . . In Taiwan and Japan, there’s a lot of promotion and education, and they treat LTC as the job of angels . . . But this is not the case in Hong Kong (Nurse)”.
Adding to the devaluation and demoralization of the workforce is the general perception of LTC facilities as a work environment lacking in professional development. Although professionalism means different things to those in different sectors, the informants in this study put forward a list of key elements for the LTC sector, including educational programs to prepare LTC professionals, specific aspects of training in LTC (e.g., gerontology, management, psychosocial skills, activity, and age-friendly interior designs), neat and tidy uniforms, a clear career ladder, competitive employment terms, and social recognition. Among all, informants were highly critical of the “low pay,” which they felt did not reflect the complexity and difficulty of their work. The job of a security guard was commonly cited for comparison: “We work so hard, doing heavy lifting, and handling pee and poop. A security guard is better off, sitting there comfortably, and not doing dirty work” (Nurse). Informants also stated that the lack of career advancement opportunities hindered the younger generation from entering the sector, and emphasized the need to transform the sector into a profession:"The LTC sector can attract new entrants only if it is professionalized . . . Just imagine, there is an educational program to prepare LTC professionals, and now you’ve completed the program, you’re a professional and because of this, you get a professional’s pay, and then people will respect you" (Administrator).
Support Systems
In facing a complicated and challenging work environment, it was noted that the informants could draw on individual, collegial, and management support systems.
Individual
Bonds formed with residents constituted the core of the individual support received by the informants. Most informants spoke of joyful occasions of “joking around” with residents, and having residents share life experiences with them. For some, the purpose of going to work was not just to do a job but also to be with “family members.” Some informants attributed the sense of connection that they cultivated with residents to the long-term nature of LTC:"When I was working in a hospital, I came to realize that a patient had been discharged before I could remember his/her name! Here, in contrast, we’re so close with the residents, like family members, and I’m happier with that" (Nurse).
Another important aspect of individual support was the sense of achievement generated from LTC work. The informants took pride in the knowledge that their work was important to the well-being of the residents: “I’m happy that I can serve the residents so that they look clean and tidy and feel comfortable” (PCW). The sense of achievement was further built on the enhancement of their knowledge about LTC. The informants applied their knowledge not only to caring for the residents but also to helping members of their own family, and this further boosted their sense of achievement: “My mom choked when eating some bread, and I gave her back blows . . . I was able to use what I’d learned from my job to help her—I’m so proud of myself” (CNA).
Collegial
Collegial support emerged as another important source of support for LTC staff. Against the high job demands in the facilities, mutual support and cooperation were noted among PCWs: “When we all finish our own work, we take a rest together, and if one of us is not done yet, we help that person get the job done before taking a rest together” (PCW). The management of unpredictable incidents, particularly health emergencies, was another occasion when team collaboration occurred: “We work as a team to take good care of the residents. Whenever there is a problem, everyone shows up to offer some help” (Nurse).
Collegial support also took the form of sharing experiences and transferring knowledge. For instance, those who had learned more standard personal care skills in another workplace were happy to teach unskilled peers through examples. CNAs played a critical role in supporting the nurses, and knowledge was often transferred between the two groups: “A nurse once told me that she could teach me the English term for one particular drug every workday” (CNA).
Management
Two crucial issues relating to system-level management support were identified. First, the informants acknowledged that their administrators had actively introduced measures to facilitate job adaptation and processes of care at work. Flexible working arrangements (e.g., allowing staff to choose between taking care of male or female residents) were made to enhance the ability of new entrants to adapt to their jobs. Management support in the form of problem resolution was also noted, with informants describing the administrators as being readily available for advice, and willing to offer timely and hands-on support when problems (e.g., residents’ complaints) occurred. Other than this, informants reported having many opportunities to relay views and feedback to the management team in staff meetings, and this contributed to a sense of self-worth: “The manager is willing to listen to us and accept our views. So I feel like I am still useful for something” (PCW).
Efforts on the part of management to help the staff to develop work goals and potentialities were also acknowledged. Programs were organized for the staff to reflect on the meaning and value of their work, which the informants found to be inspiring. In addition, the informants reported being facilitated to advance their knowledge and skills through attending in-house workshops or seminars delivered by external specialists (e.g., counselors). A study allowance in the form of time and/or funding support was also granted to staff who were willing to develop their potential in LTC.
Discussion
This study offers an in-depth and comprehensive account of the nature of the work environment and its impact on the workforce problem. Among the various aspects of the work environment, what merits further discussion is the presence of derogatory connotations that devalue and demoralize the work environment, and the role of management support in transforming the LTC sector into a profession.
The Work Environment in LTC Facilities: A Socially Constructed Concept
Complicating this already difficult LTC work environment are the critical challenges to the workforce from a societal hegemony that constructs LTC facilities as a work environment characterized by stigma, ageism, and a low potential for professional and career advancement. In addressing the workforce issue, previous studies tended to focus on the turnover and retention of existing staff, whereas this study has emphasized the impact of the work environment on existing staff, and pointed to certain deficiencies in that environment as an explanation for the paucity of new entrants. In this study, social stigmatization denotes a social process that undermines the attractiveness of the work environment. Whereas past studies highlighted various aspects of the stigmatization of those living in LTC facilities (Dobbs et al., 2008), this study highlights aspects of the stigmatization of the work environment and their impact on those working or looking for a job in this setting. Stigmatizing labels demoralize the workplace and devalue the meaning of LTC work. The stigmatization of the residents results, in part, from the functioning of these facilities as places that may promote stigma and ageism, as seen in such descriptions of them as “a stigmatizing setting” (Dobbs et al., 2008) or a “waiting house” (Tuckett, 2007), with those working in the facilities often regarded indiscriminately as “promoters” of such stigma. How LTC facilities are portrayed in the media also influences public views (Miller, Livingstone, & Ronneberg, 2017).
The findings in this study suggest that an immensely important way to destigmatize the work environment is to professionalize the sector, enabling LTC workers to gain a professional’s pay as well as the respect of the public. Yet, the results of this study indicate that there is no fertile soil for the growth of professionalism in LTC. Previous studies have consistently found that LTC nurses lack professional development opportunities in aged care (Dwyer, 2011). Without professionalism, LTC facilities will continue to be perceived and experienced as places where the jobs are of low pay and status, and that offer little potential for career development.
The Role of Management Support
Previous studies have observed that aspects related to staff–resident bonding, a sense of pride or self-achievement (Secrest et al., 2005), mutual support and cooperation (Gao et al., 2014), and the exchange of knowledge and experience among coworkers (van Beek et al., 2011) influence staff outcomes. This study demonstrates that management can act to help LTC workers counteract their highly complicated and challenging work environment, and to prop up individual and collegial support systems.
Management measures for “adaptation and facilitation at work” identified in this study have parallels in previous studies, such as job adaptation (Menne, Ejaz, Noelker, & Jones, 2007), problem resolution (Chu et al., 2014), and staff involvement in decision making (Zhang, Punnett, Gore, & The CPH-NEW Research Team, 2014). Management efforts on “the development of work goals” essentially underpin the individual and collegial support systems. Measures promoting self-values and work commitment such as the “life education” programs identified in this study have attracted scholarly attention, such as a “spirit at work in LTC” program (Kinjerski & Skrypnek, 2008). The revitalization of such values as self-esteem and morale through this sort of program could help the staff to find more meaning in their work at the individual level. The cultivation of team spirit in the same program would, in turn, reinforce the effectiveness of the support system at the collegial level.
However, in this study, a couple of issues revealed a major limitation of the management support system—the obvious lack of an LTC vision and strategic plan, including a plan to develop long-term strategies to attract new entrants. For one thing, the management tended to focus more on reactive support (e.g., dealing with “difficult family members”) instead of proactive support (e.g., developing more family-oriented care philosophies and policies) as well demonstrated in previous studies (Baumbusch & Phinney, 2014; Curry, Walker, Hogstel, & Walker, 2007; Hamann, 2014; Persson, 2008). For another, the management effort to help staff with the “development of potentialities” had little to do with career development and, thus, could hardly attract new entrants.
Implications for Policy and Practice
The phenomenon of workforce shortages cannot be addressed without considering the wider labor and education policies and the LTC sector’s infrastructure that account for the societal devaluation and demoralization of the work environment. This study highlights the importance of transforming the LTC sector into a profession, and uncovers crucial elements for achieving this, for instance, by mandating specific aspects of training and creating structured career paths and ladders for LTC workers. Previous studies have acknowledged the importance of developing competencies and skills beyond health care, such as conflict resolution (Montoro-Rodriguez & Small, 2006) and supervision (Siegel, Young, Mitchell, & Shannon, 2008), and of pursuing evidence-based educational strategies to train LTC nursing professionals (Cramer et al., 2014). However, although professional workers with the capacity to contribute to LTC, such as nurses, social workers, physiotherapists, gerontologists, psychologists, and designers, are “out there” serving different sectors, their work is disarticulated when it comes to the LTC sector. Institutions of higher education are also training those specialists, but a curriculum to train professional LTC workers is lacking. Facility administrators should strive to become involved in formulating education policies to train professional LTC workers in multiple competencies and skills. Policy makers should be convinced of the importance of regarding the LTC sector as a professional setting where the older adults are served, in much the same way as the hospital sector is recognized as a setting where different specialists work together to serve patients. Labor and education policy change needs to be accompanied by infrastructure reform (e.g., the making of new rules) that aims to enhance service and staffing standards, and to facilitate structural and functional changes at the facility level so that LTC professionals can put theory into practice. Social recognition and decent pay and employment terms for LTC workers will follow with the rise of the professionalism associated with such efforts.
Limitations and Future Research
There are three major limitations to this study. First, the majority of the interviews were conducted in the informants’ workplace, and the setting might have discouraged informants from revealing their “true” experiences of working in LTC facilities. Another limitation is the cross-sectional nature of the study, in which data were collected at one point in time. The study falls short in capturing the constant changes in the work environment influencing LTC staff outcomes. The last concerns with the transferability of the study, in which the findings may not be able to be applied to other countries with different health care systems, and political, economic, and social climates. Nevertheless, a rigorous and structured approach was adopted to guide the processes of inquiry and analysis. Further research is needed to explore issues related to professionalism in the LTC sector, so as to contribute empirical evidence to inform future initiatives and reform efforts.
Conclusion
Using the phenomenological method to explicate the lived experiences of staff working in LTC facilities has resulted in a conceptual framework that captures a wide variety of attributes of the work environment, with the potential to have a significant impact on the workforce issue. The work environment in LTC facilities not only is characterized by organization- and job-related attributes but also is a socially constructed concept, the present meaning of which hinders staff recruitment and retention. Concerted efforts based on a vision and strategic plan for LTC, particularly those from facility administrators and policy makers, are needed to improve the quality of the LTC work environment. Future initiatives should be directed toward nurturing the development of professionalism in the sector for the long-term well-being of the workforce as well as of the older adults.
Footnotes
Acknowledgements
We are grateful to all of the informants who participated in our research and to the anonymous reviewers of this article for their insightful comments.
Authors’ Note
Human Subjects Approval Number: HE30May2014-RGC2014/25.
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Research Grants Council of Hong Kong (UGC/FDS16/M01/14).