methods are necessary to the high-quality solutions of real
NRPs that involve many constraints from hospital, patients
and nurses, due to their difficulty in handling highly con-
strained problems.
Given the advantages and disadvantages of exact algo-
rithms and heuristics, some researchers applied hybrid
methods to solve NRP, by combining the assets of different
methods. Substantial results have been presented in recent
years. Burke et al. [17] proposed a two-stage hybrid
method of IP and variable neighborhood search (VNS).
This approach gained better solutions when compared with
a genetic algorithm (GA) method from ORTEC’s Harmony
[30] and a hybrid VNS approach based on heuristic ranking
method [31]. Moreover, the results showed that it outper-
formed pure IP method or pure VNS method. Tsai and Li
[18] developed a two-stage mathematical modeling and
applied GA in the two-stage processing. Their results
showed that GA is an efficient tool for the NRP and this
model could be easily modified to suit different cases. Bai
et al. [22] formed a hybrid EA combining stochastic
ranking and simulated annealing method for a classical
NRP problem [32], in which there was one hard constraint
and three soft constraints. They compared the hybrid EA
approach with other four approaches (TSHH [33], IGA
[34], EDA [35] and SAHH [36]) and found that the hybrid
EA approach obtained better performance for the NRP
instances.
As the scale of a nurse rostering problem increases
(more nurses and longer scheduling period) and the con-
straints become more complex, research on large-size nurse
rostering problems is greatly needed. As a result, several
studies on large-size NRPs such as CNRP [21] have been
presented. Yet, we wish to find an approach to improve
upon this research. Our research would handle a complex
nurse rostering problem [21] proposed from hospitals in
China. This paper will present an EA for solving CNRP, a
large-scale NRP with many complex constraints. The
CNRP possesses two main features: comparatively large
number of nurses and an additional constraint of balancing
the nurses’ workload. Therefore, the proposed results will
help further the research on solutions for large complex
rostering problems.
3 The nurse rostering problem
CNRP, a large-size nurse rostering problem with complex
constraints, will be introduced in this section. In the nurse
shifting system of hospitals [20, 21], the day shifts can be
classified into three types, as follows:
A-shift: 8:00–15:00
P shift: 15:00–22:00
N-shift: 22:00–8:00
A nurse works at most one shift per day. The goal of the
problem is to come up with a shifts-assignment solution.
CNRP contains several hard and soft constraints. The hard
constraints should be satisfied, including:
HC1 The number of nurses should satisfy daily coverage
requirements for each shift type.
HC2 The number of total working days for each nurse
should range between the maximum boundary and the
minimum boundary.
HC3 An N-shift followed by an A-shift is not allowed.
The soft constraints are those to be satisfied as much as
possible, which also serve as the criteria for evaluating the
quality of the solution. The soft constraints are described as
follows:
SC1 (Fair workload) The difference between the number
of different working shifts for each nurse and the
corresponding average value should be no more than 1.
SC2 The number of consecutive working days of each
nurse should range between three and seven.
SC3 There should be at most five consecutive working
night shifts for each nurse.
SC4 (Complete weekends) There should be either no
shift or two shifts on a weekend.
SC5 There should be at most four working days on
weekends in the scheduling period.
SC6 There should be at least 2 days of rest after a series
of working days.
According to the requirement [21
], the first four soft
constraints have the same priority and the last two soft
constraints have lower priority. All of the soft constraints’
priorities correspond to the soft constraints’ weights in
quality evaluation. According to the complete statement of
the problem above, we present a mathematical model of
CNRP, which is similar to the one raised by Burke [17].
The NRP contains a task of shift assignment of M days’
scheduling period which involves N nurses. Let I be the set
of nurses, J be the set of days in the period and K be the set
of nurses’ shift types. The decision variable x
ijk
denotes
whether nurse i works on the shift k in day j, in which:
k ¼
1; works on A shift
2; works on P shift
3; works on N shift
8
<
:
Hence,
x
ijk
¼
1; nurse i works on the shift k in day j
0; otherwise
;
where i 2 I and j 2 J.
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