Journal Search Engine
Search Advanced Search Adode Reader(link)
Download PDF Export Citaion korean bibliography PMC previewer
ISSN : 1598-7248 (Print)
ISSN : 2234-6473 (Online)
Industrial Engineering & Management Systems Vol.18 No.1 pp.1-24
DOI : https://doi.org/10.7232/iems.2019.18.1.001

Planning and Scheduling of Manpower in an Annualized Hours Environment Integrating Workers Willingness and Efficiency

Md. Gulzarul Hasan*, Zoha Qayyum, Syed Suhaib Hasan
Department of Statistics and Operations Research, Aligarh Muslim University, Aligarh, India
Corresponding Author, E-mail: gulzarhsn@gmail.com
April 20, 2017 September 27, 2017 September 28, 2017

ABSTRACT


We have considered the problem of Annualized hours (AH) in Manpower planning. AH is a method of distributing working hours with respect to the demand over a year. The contribution of this paper is to propose basic models of Annualized hours planning problem with a new objective and the integration of workers willingness to work on each task and week. Four mathematical programming formulations are presented. These proposed models are the best fit when there is a need for minimizing capacity shortage of multiple tasks with preference weights. These models will help the decision makers to plan and schedule their workers in an AH environment with a lot of results of overtime, undertime, annual and weekly effective working hours by each employee, shortages in each week, to manage the flexible working of willingness etc. Finally, An experimental study is performed to validate and analyze the effectiveness of these mathematical programming models. Results of different models are compared and it has been found that working flexibility is directly proportional to the capacity shortages of employees. There is a proper need for control and balance of flexibility.



초록


    1. INTRODUCTION

    Manpower planning plays a key role in the success of almost every organization especially production and service industries. It increases their efficiency and helps them to achieve targets. The major reasons for its need are organizational goals, workers preferences, work regulations, collective bargaining, and legal constraints.

    Flexible working is an efficient way of managing manpower by providing better employee satisfaction. It increases workers productivity and engagement and provides flexible retirement, attraction, and retention of senior executives etc. (See, Direction, 2008;Report, 2012;Wheatley, 2016;RARC Report, 2015;Qin et al., 2015).

    Annualized hours policy is one of the methods of flexible working that helps to schedule and calculate working time throughout the year. Also, it helps the organization to efficiently match workforce demand and it’s availability over the year. The variation in demand may be due to seasonal effects whereas the variation in workforce availability is due to illness, training, vacation etc. As high demand leads to hectic overtime and less demand leads to underutilization of the workforce, there is a need for proper management. AH policy provides effective plans to manage better work-life balance, less overtime, improved working flexibility and minimizing capacity shortages during peak requirements (Hung, 1997;RARC Report, 2015;Bell and Hart, 2003;Ryan and Wallace, 2016).

    Earlier applications of AH were prevalent among several French, German and Scandinavian companies (Teriet, 1977;Lynch, 1995;Gall, 1996;Tucker et al., 2001). Later, it is as used with different labor legislation and collective bargaining of countries like France (Grabot and Letouzey, 2000), Britain (Rodriguez, 2003) and Kouzis and Kretsos (2003). A brief list of wide implementation and labor legislation of AH can be found in Hung (1997), Kouzis and Kretsos (2003), Bell and Hart (2003). It gives greater flexibility by distributing working hours of the hired workforce over the year to fulfill the demand fluctuation. The greater advantage of Annualized hours policy is that it leads to a reduction in overtime, underutilization of workforce, total cost, part-time workers and gives improved service level. It gives fruitful results in labor-intensive industries for eg. Filho and Marçola (2001) reduces overtime by 94% and the use of temporary workers by 53%. Reduction in temporary workers gives improved service level in Thomas Sanderson (Workforce Logistics, 2016) productivity increased by 25% and labor cost reduced by previous year costs. Also, McMeekin (1995) in Tesco Distribution showed that the stock levels had reduced to a large extent on the implementation of AH policy.

    The layout of this paper is as follows; Section 2 discusses related literature; In Section 3 presents problem motivation and contribution; Section 4 describes the problem and its mathematical programming formulation; Section 5 is about the experimental study for proposed models of Annualized hours planning problem (AHPP). Finally, section 6 is the conclusion.

    2. LITERATURE REVIEW

    Manpower planning and scheduling in an AH environment is well studied by many authors. Some of them are Hung (1999a, 1999b), Filho and Marçola (2001), Azmat and Widmer (2004), Azmat et al. (2004), Corominas et al. (2002), Corominas et al. (2007b), Lusa et al. (2008a, 2008b), Hertz et al. (2010), Corominas and Pastor (2010), Lusa and Pastor (2011), Van der Veen et al. (2012), Sureshkumar and Pillai (2012, 2013), Van der Veen et al. (2014) and Hasan and Hasan (2017). Detailed Characterization and classification of the annualized working hours planning problems are presented by Corominas et al. (2004a).

    Hung (1999a, 199b) proposes heuristics for determining minimal manpower and their scheduling in single and multiple shifts over the year. Azmat and Widmer (2004) present a three-step heuristic approach for manpower scheduling with minimal staff determination, overtime estimation, and holiday consideration. Most of the models in AH planning and scheduling problems are based on mathematical programming.

    A brief Summary of mathematical models is presented through table.

    3. PROBLEM MOTIVATION AND CONTRIBUTION

    Very limited work is present in literature for Planning and scheduling problems in an Annualized hours environment. This work is an extension of previously proposed mathematical programming models for multiple tasks by Corominas et al. (2007c), Sureshkumar and Pillai (2012, 2013) and for the single task by Lusa et al. (2008a, 2008c). In these models, the problem objective for single task problem is to minimize the sum of maximum and average of relative capacity shortages. While for multiple task problem the objective is to minimize the sum of maximum relative capacity shortage among all the tasks and an average of relative capacity shortages of all tasks.

    The objective function for Single Task:

    Min: Z = α × S + ( 1 α ) × t T S t D t total weeks

    The objective function for Multiple Tasks:

    Min:  Z = α × S ' + ( 1 α ) × j J t T S j t D j t ( total weeks ) × ( total tasks )

    where S and S′ are maximum relative capacity shortage for single and among multiple tasks with weight α.

    Proposed Objective function:

    Min:  Z = β j × Z j

    where,

    j J   β j = 1

    and

    Z j = α j × S j + ( 1 α j ) × t T S j t D j t total weeks

    where Sj is maximum relative capacity shortage for task j with weight αj. The proposed objective helps for better decision making with respect to the preference of minimizing capacity shortages for a particular task. It is the weighted sum of relative capacity shortages for multiple tasks.

    In addition to the new objective, the problem is analyzed by introducing workers willingness to work on multiple tasks and weeks. The performance of workers is determined by many factors such as Age, experience, intelligence, mental and physical strength, working conditions, work-life balance, attitude, job type etc. (see, Blumberg and Pringle, 1982;Riggs, 1981;Waldman and Spangler, 1989;Sjöberg, 1994). Recently the concept of workers willingness or readiness is introduced in the planning and scheduling of workers by Othman et al. (2012) and Yang et al. (2016). They used it as a job satisfaction factor of workers. Both Othman and Yang considers workers willingness to work on different machines in a particular time period t. Here we introduced this concept for working in different weeks. It provides greater flexibility to the workers. Previous work in AH planning only considers the relative efficiency of workers on each task. Here, we considered the willingness in two different forms i.e. willingness to work on different tasks and to work in each week. The willingness is determined over a scale of 1, if willingness is 0.3 then it means that the worker is 30% ready to work.

    The objectives of the proposed models are:

    • (1) Minimize relative capacity shortage with respect to demand as per the preference of Decision Maker for each task.

    • (2) To determine the scheduled working hours for each employee.

    • (3) To analyze the model by providing more flexibility to the workers through willingness and annualized hours policy.

    4. PROBLEM FORMULATION

    In the present work, we are working on a basic model of AH planning problem and optimizing the service level by taking into account workers working efficiency and willingness. No temporary workers are considered. Overtime is allowed with an upper bound. It is assumed that there are multiple tasks and that the company forecasts the demand and establishes the capacity requirement. As overtime is bounded, so the capacity shortage is possible in certain weeks. To make the model more real, crosstrained workers with willingness to work on each task is considered with different relative efficiency (RE) (scale of 1, for eg. a value of 0.6 means that such a worker needs to work 1/0.6 h to perform a task that a worker with a RE equal to 1 would perform in 1 hour) and willingness (scale of 1) associated to them. The number of weekly working hours fall within an interval with upper and lower bound. Also, the total working hours for the planning period lies within an interval. Four Mathematical programming formulations M-1, M-2, M-3, and M-4 are presented. Where M-1 considers new objective and relative efficiency, M-2 considers workers willingness to work in each week in addition of new objective and relative efficiency, M-3 considers workers willingness to work on each task with new objective and relative efficiency and M-4 in addition of M-3 takes account of workers willingness to work in each week of the year.

    The problem studied and modeled in this paper is based upon the minimization of the relative capacity shortage with respect to demand of workforce. Since, high relative capacity shortage leads to worse service level, hence the objective of the problem is to minimize.

    The characteristics of the problem are as follows:

    • (1) Planning period of the allocation of the workforce is taken as 52 weeks (i.e 1 year).

    • (2) Holiday weeks are predetermined by workers.

    • (3) Working hours for each week and year is lower and upper bounded.

    • (4) Multiple tasks with forecasted demand are taken with the assumption that the staff is multitasking having different relative efficiency associated to them.

    • (5) Workers willingness for each task is considered.

    • (6) Overtime is permitted with an upper bound.

    • (7) Hiring temporary workers is not allowed.

    • (8) The average number of working hours for a group of 12 consecutive weeks cannot be larger than 44 hours per week.

    • (9) A Utility function is to be optimized.

    The list of notations which will be used throughout the paper is given below:

    Decision variables and Model parameters

    • I set of workers indexed by i.

    • J set of tasks indexed by j.

    • T set of week indexed by t.

    • Djt demand for task j in week t.

    • Lit, Uit minimum and a maximum number of allowed working hours for worker i in week t.

    • Li, Ui minimum and a maximum number of working hours allowed for worker i in entire.

      planning horizon T i.e. a year.

    • REij relative efficiency of worker i on task j.

    • WTij worker i willingness to work on task j.

    • WWit worker i willingness to work in week t.

    • Aijt an effective number of working hours by worker i on task j during week t.

    • Ait an effective number of working hours by worker i during week t.

      i.e. { A i t = j J A i j t , i I , t T }

    • Ai the number of effective working hours by worker i during planning period T.

      i.e. { A i = j J t T   A i j t , i I }

    • Yijt the number of working hours by worker i with full efficiency on task j.

      during week t.

    • Yit the number of working hours by worker i with full efficiency during week t.

      i.e. { Y i t = j J Y i j t , i I , t T }

    • Sj maximum relative capacity shortage of workers with respect to demand Dj for task j

    • Sjt capacity shortage of workers for task j in week t.

    • αj the weight associated with the maximum relative capacity shortage of task j.

    • βj the weight associated to the sum of the maximum and average relative capacity shortage of each task j.

    • Z the weighted sum of relative capacity shortages Zj for all tasks.

    4.1 Mathematical Programming Models

    The mathematical programming models of above explained problem are as follows:

    M-1 model:

    This model minimizes the new objective considering the relative efficiency of workers on each task.

    Objective function:

    Min : Z = β j × Z j

    where,

    j J   β j = 1

    and

    Z j = α j × S j + ( 1 α j ) × t T S j t D j t total weeks

    Subject to,

    Maximum relative capacity shortage:

    S j S j t D j t j J , t T
    (1)

    The maximum relative capacity shortage is greater than the weekly relative capacity shortages.

    Annual working hour constraint:

    L i t T Y i t U i i I
    (2)

    The Annual Sum of weekly working hours to be allotted to each worker with full efficiency is bounded by the minimum number of working hours and the maximum number of working hours which includes overtime hours.

    Weekly working hour constraint:

    L i t Y i t U i t i I , t T
    (3)

    Weekly working hours to be allotted to each worker with full efficiency is bounded by the minimum number of working hours and the maximum number of working hours.

    Demand constraint:

    i I R E i j × Y i j t + S j t D j t j J , t T
    (4)

    The number of working hours allotted to the workers for each task and for each week added to the shortage hours must be greater and equal to the forecasted demand.

    Effective hours constraint:

    A i j t = R E i j × Y i j t i I , j J , t T
    (5)

    Aijt is the number of effective working hours by the worker i for each task j and week t after considering their relative efficiency.

    Average working hour constraint:

    t = m L + 1 j Y i t h L × L   i I , m = L , ... , T
    (6)

    It imposes an upper bound on the average weekly working hours for any consecutive L working weeks.

    Non-negativity restrictions:

    Y i t 0 i I , t T
    (7)

    and

    A i j t 0 i I , j J , t T
    (8)

    The rest of the three models M-2, M-3, and M-4 are same as M-1, except the change in demand and effective hours constraint.

    M-2 model:

    Demand constraint:

    i I R E i j × Y i j t × W W i t + S j t D j t   j J , t T
    (9)

    The number of working hours allotted to the workers, after including their efficiency and willingness to work in each week added to the shortage hours, must be greater and equal to the forecasted demand.

    Effective hours constraint:

    A i j t = R E i j × Y i j t × W W i t   i I , j J , t T
    (10)

    A i j t is the number of effective working hours by the worker i for each task j and week t after considering relative efficiency and willingness to work in each week.

    M-3 model:

    Demand constraint:

    i I R E i j × Y i j t × W T i j + S j t D j t j J , t T
    (11)

    The number of working hours by the workers, after including their efficiency and willingness to work on each task added to the shortage hours, must be greater and equal to the forecasted demand.

    Effective hours constraint:

    A i j t = R E i j × Y i j t × W T i j i I , j J , t T
    (12)

    Ajt is the number of effective working hours by the worker i for each task j and week t after considering relative efficiency and willingness to work on each task.

    M-4 model:

    Demand constraint:

    i I R E i j × Y i j t × W T i j × W W i t + S j t D j t j J , t T
    (13)

    The number of working hours allotted to the workers, after including their efficiency, willingness to work on each task and week added to the shortage hours, must be greater and equal to the forecasted demand.

    A i j t = R E i j × Y i j t × W T i j × W W i t i I , j J , t T
    (14)

    Aijt is the number of effective working hours by the worker i for each task j and week t after considering relative efficiency, willingness to work on each task and week.

    5. EXPERIMENTAL STUDY

    To show the applicability and decision aspects of our proposed models, we consider a hypothetical manpower planning problem. Suppose a production firm needed workers for two tasks (Task-1 and Task-2) for a period of a year. The demand for each task in terms of working hours is forecasted with different patterns and presented in Figure 1. Demand for Task-1 and Task-2 follows different patterns. A staff of 13 workers is considered to fulfill the total demand. These workers are cross-trained with the different relative efficiency of working on each task. To provide flexibility to the staff, the firm asks the workers for their willingness to work on each task and in each week. The firm restricts the workers by allowing a maximum of 3 weeks for 0.3 (30%) and 4 weeks for 0.8 (80%) willingness without holidays. Two consecutive holiday weeks are allowed in summer and winter season only without willingness to work in particular week. The firm needs the following results for better manpower planning in an AH environment:

    • (1) To schedule workers throughout the year.

    • (2) The firm wants to minimize more the relative capacity shortage of Task-2 as compare to Task-1.

    • (3) To determine the relative capacity shortages, effective hours in each week, annual effective working hours, overtime and undertime by the employees.

    To analyze these results four cases are considered:

    • Case-1: It works on model M-1 and takes account of only relative efficiency of workers.

    • Case-2: It considers relative efficiency and willing-ness of workers to work in each week and uses model M-2.

    • Case-3: It considers relative efficiency as well as the willingness of workers to work on each task and uses model M-3.

    • Case-4: It utilizes model M-4 and in addition of relative efficiency, workers willingness on each task, it considers workers willingness of working in each week of the year.

    The data for the problem is presented below:

    • Number of Workers (I)   13 workers

      Number of Tasks (J)   2 Tasks

      Planning horizon (T)   52 weeks (approx. a year)

      Minimum and maximum no. of weekly working hours    35, 50 hours

      i.e. L i t ,    U i t i I ,    t T

      Minimum and maximum no. of annual working hours   1920, 2100 hours

      i.e. L i ,    U i i I

      α j   j J    0.99, 0.99

      β j   j J    0.1, 0.90

      Relative efficiency (RE)     See Table 1

      Workers willingness to work on each task (WT)   See Table 1

      Workers holiday weeks    See Table 1

      Workers willingness to work in each week (WW)   See Table 2

        h L    44 hours

      L    12 weeks

    The relative efficiency, holidays and willingness to work on each task with respect to the workers is presented in Table 1 and willingness to work in each week is in Table 2.

    These models M-1, M-2, M-3 and M-4 are modeled in AMPL language and solved using BARON solver using NEOS server online facility provided by Wisconsin Institutes for Discovery at the University of Wisconsin in Madison for solving Optimization problem (see, Czyzyk et al., 1998;Dolan, 2001;Gropp and Moré, 1997;NEOS Server, 2016).

    5.1 Results

    The Proposed models are solved and the results are obtained. The proposed models minimize the demand and supply gap with preference to a particular task as compared to other tasks in an AH environment considering flexible working. The computation time by solvers is found acceptable and gives optimal results within seconds for all models. Since the decrease in the sum of relative capacity shortage and maximum relative capacity shortage gives an increase in service level. Here, for this problem, the decision maker is interested to minimize shortages of task-2 as compared to task-1. Since the data of results is too much, The results are presented through figures. The demand, supply, and shortage in each week for task-1 and 2 is presented in Figure 2. Task-1 shows high shortages while task-2 have no shortages in all cases. As we move from case-1 to case-4, the capacity shortage in different weeks for task-1 increases, while for task-2 is zero. In case-1 during some weeks, the supply of working hours is higher than the demand. Total annual scheduled allocated working hours, Overtime and undertime by each employee is given in Figure 3. The negative values are the undertime hours by the workers, i.e. below contracted 1920 hours. As flexibility increases effective hours decreases by increasing the undertime i.e. lesser than 1920 hours. Figure 4 shows the capacity shortage in each week for all cases. The maximum variation in capacity shortage can be seen in case-4 while minimum variation lies in case-1. In some weeks capacity shortage of task-1 is high for case-1 as compared case-2 and case-3 as compared to case-4, although the total annual capacity shortage increases from case-1 to case-4. The values of maximum relative capacity shortages and the annual sum of relative capacity shortages is presented in Figure 5. It shows that as the flexibility to working increases from case-1 to case-4, there is an increase in maximum relative capacity shortages and total relative shortages over the year. In Figure 6, there is a fewer increase in total annual capacity shortages from case-1 to case-2 but a high increase from case-2 to case-3 and case-4. It shows that willingness to work in weeks do not affect the shortages more as compared to the willingness of task. There is a higher increase when we consider the willingness of task (case-3). The scheduled working hours on each task for case-1, 2, 3 and 4 are presented in Appendix I, II, III and IV.

    where, M a x S 1 = S 1 , M a x S 2 = S 2 , R E S U M 1 = t T S 1 t D 1 t , R E S U M 2 = t T S 2 t D 2 t

    6. CONCLUSION AND DISCUSSION

    The contribution of this paper is to propose mathematical programming models of Annualized hours planning problem with a new objective and the integration of workers willingness to work on each task and week. This paper considers a new flexibility factor of willingness to work in weeks. Four mathematical programming formulations (M-1, M-2, M-3, and M-4) are presented in section 4. These proposed models are the best fit when there is a need for minimizing capacity shortage of multiple tasks with preference weights for different tasks. These models will help the decision makers to plan and schedule their workers in an AH environment with a lot of results of overtime, undertime, annual and weekly effective working hours by each employee, shortages in each week, to manage the flexible working of willingness etc.

    Also, a hypothetical experimental study has been done to show the effectiveness of the proposed model. For better understanding, an analysis has been done on a problem of two task demand scenario through four cases. As the data is huge with respect to paper writing, the summary of the optimal solution obtained after solving the models are presented by figures and tables. Results of the problem show that as flexibility increases the relative capacity shortage increases from case-1 to case-4. In some weeks the total capacity shortage for case-2 is high than case-1 and case-3 is high than case-4. Only Task-1 shows shortages, task-2 do not show any shortage to all cases. Surely, flexibility helps employees to better manage their work-life balance but high shortages cost to employers. So, there is a need for proper control and balance of flexibility. Model-1 performs best as do not considers the flexibility concept of willingness to work on tasks and in weeks. This work can be extended by considering stochastic or fuzzy uncertainties. Since uncertainty always lies in real life problems, so there is a need for planning and scheduling study in AH environment.

    Figure

    IEMS-18-1-1_F1.gif

    Two task demand pattern.

    IEMS-18-1-1_F2.gif

    Demand coverage and shortage chart for cases 1, 2, 3, 4.

    IEMS-18-1-1_F3.gif

    Annual worked hours, overtime and undertime chart for all cases.

    IEMS-18-1-1_F4.gif

    Total weekly capacity shortage chart for cases-1, 2, 3, 4.

    IEMS-18-1-1_F5.gif

    Model objectives, maximum and average relatives capacity shortages.

    IEMS-18-1-1_F6.gif

    Total annual capacity shortage for cases-1, 2, 3, 4.

    Table

    Worker s relative efficiency, willingness to work on each task and holiday weeks

    Worker’s willingness to work in different weeks

    Workers annual schedule on task-1 and 2 for case-1

    Workers annual schedule on task-1 and 2 for case-2.

    Workers annual schedule on task-1 and 2 for case-3.

    Workers annual schedule on task-1 and 2 for case-4.

    REFERENCES

    1. Azmat, C. S. and Widmer, M. (2004), A case study of single shift planning and scheduling under annualized hours: A simple three-step approach, European Journal of Operational Research, 153(1), 148-175.
    2. Azmat, C. S. , Hürlimann, T. , and Widmer, M.(2004), Mixed integer programming to schedule a single shift workforce under annualized hours, Annals of Operations Research, 128(1-4), 199-215.
    3. Bell, D. N. and Hart, R. A. (2003), Annualised hours contracts: The way forward in labour market flexibility?, National Institute Economic Review, 185(1), 64-77.
    4. Blumberg, M. and Pringle, C. D. (1982), The missing opportunity in organizational research: Some implications for a theory of work performance, The Academy of Management Review, 7(4), 560-569.
    5. Corominas, A. and Pastor, R. (2010), Replanning working time under annualised working hours, International Journal of Production Research, 48(5), 1493-1515.
    6. Corominas, A. , Lusa, A. , and Olivella, J.(2012), A detailed workforce planning model including nonlinear dependence of capacity on the size of the staff and cash management, European Journal of Operational Research, 216(2), 445-458.
    7. Corominas, A. , Lusa, A. , and Pastor, R.(2002), Using MILP to plan annualised working hours, Journal of the Operational Research Society, 53(10), 1101-1108.
    8. Corominas, A. , Lusa, A. , and Pastor, R.(2004a), Characteristics and classification of the annualised working hours planning problems, International Journal of Services Technology and Management, 5(5-6), 435-447.
    9. Corominas, A. , Lusa, A. , and Pastor, R.(2004b), Planning annualised hours with a finite set of weekly working hours and joint holidays, Annals of Operations Research, 128(1-4), 217-233.
    10. Corominas, A. , Lusa, A. , and Pastor, R.(2007a), Using a MILP model to establish a framework for an annualised hours agreement, European Journal of Operational Research, 177(3), 1495-1506.
    11. Corominas, A. , Lusa, A. , and Pastor, R.(2007b), Planning annualised hours with a finite set of weekly working hours and cross-trained workers, European Journal of Operational Research, 176(1), 230-239.
    12. Corominas, A. , Lusa, A. , and Pastor, R.(2007c), Planning production and working time within an annualised hours scheme framework, Annals of Operations Research, 155(1), 5-23.
    13. Czyzyk, J. , Mesnier, M. P. , and Moré, J. J.(1998), The NEOS Server, IEEE Journal on Computational Science and Engineering, 5(3), 68-75.
    14. Direction, S. (2008), Flexible working as human resources strategy: Benefits to the organization and its personnel, Strategic Direction, 24(8), 9-11.
    15. Dolan, E. (2001), The NEOS server 4.0 administrative guide. Technical Memorandum ANL/MCSTM250, Mathematics and Computer Science Division, Argonne National Laboratory.
    16. Filho, E. V. G. and Marçola, J. A. (2001), Annualized hours as a capacity planning tool in make-to-order or assemble-to-order environment: An agricultural implements company case, Production Planning and Control, 12(4), 388-398.
    17. Gall, G. (1996), All year round: The growth of annual hours in Britain, Personnel Review, 25(3), 35-52.
    18. Goeminne, M. (2012~2013), An efficient heuristic for scheduling personnel under an annualized workload, Faculty of Economics and Business Administration, Ghent University, Thesis.
    19. Grabot, B. and Letouzey, A. (2000), Short-term manpower management in manufacturing systems: New requirements and DSS prototyping, Computers in Industry, 43(1), 11-29.
    20. Gropp, W. and Moré, J. J. (1997), Optimization environments and the NEOS server. In Buhmann, M. D. and Iserles, A. (eds.), Approximation Theory and Optimization, Cambridge University Press, 167-182.
    21. Hasan, M. G. and Hasan, S. S. (2017), Manpower planning with annualized hours flexibility: A fuzzy mathematical programming approach, Decision Making in Manufacturing and Services, 10(1-2), 5-29
    22. Hertz, A. , Lahrichi, N. , and Widmer, M.(2010), A flexible MILP model for multiple-shift workforce planning under annualized hours, European Journal of Operational Research, 200(3), 860-873.
    23. Hung, R. (1997), Annualized hours and aggregate planning, Production and Inventory Management Journal, 38(4),48-50.
    24. Hung, R. (1999a), A multiple-shift workforce scheduling model under annualized hours, Naval Research Logistics, 46(6), 726-736.
    25. Hung, R. (1999b), Scheduling a workforce under annualized hours, International Journal of Production Research, 37(11), 2419-2427.
    26. Kouzis, G. and Kretsos, L. (2003), Annualised hours in Europe, EurWORK, Available from: https://www.eurofound.europa.eu/observatories/eurwork/comparative-information/annualised-hours-in-europe.
    27. Lusa, A. , Corominas, A. , and Munoz, N.(2008a), A multistage scenario optimisation procedure to plan annualised working hours under demand uncertainty, International Journal of Production Economics, 113(2), 957-968.
    28. Lusa, A. and Pastor, R. (2011), Planning working time accounts under demand uncertainty, Computers and Operations Research, 38(2), 517-524.
    29. Lusa, A. , Corominas, A. , and Pastor, R.(2008b), An exact procedure for planning holidays and working time under annualized hours considering cross-trained workers with different efficiencies, International Journal of Production Research, 46(8), 2123-2142.
    30. Lusa, A. , Pastor, R. , and Corominas, A.(2008c), Determining the most appropriate set of weekly working hours for planning annualised working time, International Journal of Production Economics, 111(2), 697-706.
    31. Lynch, P. (1995), Annual hours: An idea whose time has come, Personnel Management, 46-50.
    32. McMeekin, J. (1995), Why Tesco’s new composite distribution needed annual hours, International Journal of Retail and Distribution Management, 23(9), 36-38.
    33. NEOS Server(2016), State-of-the-Art Solvers for Numerical Optimization, cited 2016 Jun. 21, Available from: http://www.neos-server.org/neos/.
    34. Othman, M. , Bhuiyan, N. , and Gouw, G. J.(2012), Integrating workers’ differences into workforce planning, Computers and Industrial Engineering, 63(4), 1096-1106.
    35. Qin, R. , Nembhard, D. A. , and Barnes II, W. L.(2015), Workforce flexibility in operations management, Surveys in Operations Research and Management Science, 20(1), 19-33.
    36. RARC report(2015), Flexibility at work: Human resource strategies to help the postal service, Report number- RARC-WP-15-004, Jan-5, 2015, Office of Inspector General, United States Postal Service.
    37. Report(2012), Future of work institute, The benefits of flexible working arrangements: A future of work report, www.hotspotsmovement.com.
    38. Riggs, J. L. (1981), Production Systems: Planning, Analysis and Control (3rd ed), John Wiley, New York.
    39. Rodriguez, M. (2003), Flexible working patterns using annualised hours, Work Study, 52(3), 145-149.
    40. Ryan, L. and Wallace, J. (2016), Annual hours, workplace partnership and mutual gains: Exploring the links, Employee Relations, 38(2), 248-266.
    41. Sjöberg, L. (1994), Arbetsmotivation i en krisekonomi: En studie av prognosfaktorer. (Work motivation in a crisis economy: A study of prognostic variables) (Studier i ekonomisk psykologi No. 121). Handelshögskolan I Stockholm: Institutionen för ekonomisk psykologi.
    42. Sureshkumar, M. R. and Pillai, V. M. (2012), Planning annualized hours when spike in demand exists, International Journal of Industrial Engineering Computations, 3(3), 313-320.
    43. Sureshkumar, M. R. and Pillai, V. M. (2013), An efficient method to reduce relative capacity shortage using annualized hours planning, International Journal of Industrial Engineering Computations, 65(1-4), 571-580.
    44. Teriet, B. (1977), Flexiyear schedules-only a matter of time, Monthly Lab. Rev., 100, 62.
    45. Tucker, P. , Gaertner, . , and Mason, C.(2001), Balancing flexibility for the employer and the employee: A case study of the development of annualized hours employment contracts, Journal of Human Ergology, 30(1-2), 77-82.
    46. Ull Hasan, M. G. , Ali, I. , and Hasan, S. S.(2016), Annualized hours planning with fuzzy demand constraint, ProbStatForum, 09, 50-56.
    47. Van der Veen, E. , Hans, E. W. Veltman, B., Berrevoets, L. M., Berden, H. J. J. M.(2014), A case study of cost-efficient staffing under annualized hours, Health Care Management Science, DOI 10.1007/s10729-014-9292-0.
    48. Van der Veen, E. , Boucherie, R. J. Van Ommeren, J. C. W.(2012), Optimal staffing under an annualized hours regime using cross-entropy optimization (Memorandum 1982, Department of Applied Mathematics), University of Twente, Enschede.
    49. Waldman, D. A. and Spangler, W. D. (1989), Putting together the pieces: A closer look at the determinants of job performance, Human Performance, 2(1), 29-59.
    50. Wheatley, D. (2016), Employee satisfaction and use of flexible working arrangements, Work, Employment and Society, 1-19.
    51. Workforce Logistics(2016), Annualised hours case study- Thomas Sanderson, cited 2016 Jun. 21. Available from: http://www.workforce-logistics.com/case-studies/thomas-sanderson/.
    52. Yang, G. , Tang, W. , and Zhao, R.(2016), An uncertain workforce planning problem with job satisfaction, International Journal of Machine Learning and Cybernetics, 1-13.