machines, each of them containing some of the original m
machines. Johnson’s algorithm is then applied to the m
1
problems
with two virtual machines and m
1
sequences are obtained. The
schedule with the minimum flowtime is selected. The CDS heuristic
has a computational complexity of Om
2
nþ mnlogn
and research-
ers have typically used CDS as benchmark for comparisons. Gupta
[14] introduced three simple heuristics, named minimum idle time
(MINIT), minimum completion time (MICOT) and MINIMAX algo-
rithms, and compared the results against the CDS heuristic provid-
ing better results with less computational time. However, it has to
be noted that the maximum instance size tested at that time was
really small with just 7 jobs and 20 machines maximum (7 20).
Krone and Steiglitz [21] presented an early heuristic in which in the
first phase, permutation sequences were improved by insertion
movements. In the second phase, job passing was allowed. Miyazaki
et al. [29] also presented a heuristic but in this case based on the
improvement of the sequence by the interchange of adjacent jobs.
Later, Miyazaki and Nishiyama [28] provided a similar extension
but for the additional consideration of job weights. Ho and Chang
[18] proposed a heuristic that works by minimizing the idle times
betweenjobsinthem machine case. The heuristic was evaluated
against other existing methods but mainly those proposed for
makespan minimization. Rajendran and Chaudhuri [37] introduced
three simple heuristics and compared them with those of Gupta
[14],Miyazakietal.[29] and the aforementioned heuristic of Ho
and Chang [18]. The results favored the introduced methods for the
studied instances. In a related work, Rajendran and Chaudhuri [36],
the same authors presented another heuristic that uses a lower
bound in the construction phase of the sequence. The proposed
heuristic is applied also to the no-wait problem. No comparisons
against the three heuristics of Rajendran and Chaudhuri [37]
are shown.
The NEH heuristic of Naw az et al. [32] is regarded as th e best
heuristic for the PFSP with makespan c riterion [40,39]. It is based
on the idea that jobs with larger total processing times should be
scheduled as early as possible. Consequently, the heuristic first
generates an initial order of jobs with respect to de scending
sums of their total processing times. Then a job sequence is
constructed by evalu ating all the sequences obtained by insert-
ing a job fro m this initial order into all the possible positions of
the cur rent partial sequence. The NEH heuristic evaluates
[n(nþ1)/21] sequences and h as a complexity of O(n
3
m)for
the TFT criterion. Due to its effectiveness, the NEH heuristic has
been inspiring research on the total completion time criterion
since its publication. Rajen dran [35] proposed an insertion
heuristic, denoted as Raj, having many simila rities with the
NEH heuristic. The heuri stic arranges the jobs according to the
weighted total processing times and inser ts a job into a
restricted subset of all possible positions of the curren t partial
sequence. A ccording to the author’s results, the p roposed heur-
istic is more efficient than the methods of Gupta [14] , Miyazaki
et al. [29] and Ho and Chang [18]. Another heuristic was
proposed by Woo and Yim [46] (denoted as WY in short). Unlike
the Raj heuristic, WY does not require an initial starting job
sequence. However, it als o has an inserti on phase where a
schedule is construct ed by inserting all non-scheduled jobs in
all possible positions of the partial sequence. This heuristic is
also based on the aforementioned NEH heuristic but has a higher
complexity of O(n
4
m). The autho rs concluded that their algo -
rithm outperforms the adaptation for flowtime minimization of
the NEH, CDS and Raj heuristics.
Framinan et al. [10] investigated the phases of the NEH
heuristic and their contribution to its excellent performance
regarding makespan minimization. They proposed to modify the
NEH heuristic in order to accomplish total flowtime criterion, and
proved that the NEH heuristic starting with an initial sequence of
jobs sorted by an increasing (instead of decreasing) sum of
processing times performs better than the adaptation of the
original NEH heuristic. It almost equals the WY heuristic in terms
of the quality of the solutions but with smaller computational
times. Later, Framinan et al. [9] further delved into the NEH
initialization and studied 177 different initial orders for the NEH,
including some specially geared towards TFT minimization.
Among the proposed methods, a heuristic called B5FT, consisting
of the best of five-tuples among the 177 approaches, is shown to
outperform the RZ heuristic of Rajendran and Ziegler [38] (to be
discussed later) and the WY method which were regarded as the
best constructive heuristics for the problem prior to the year 2000
according to Framinan et al. [11]. Framinan and Leisten [8]
presented another NEH-based heuristic, referred to as FL, with
the same complexity as the WY method. After the insertion
process in the basic NEH heuristic, the obtained partial sequence
is improved by performing a pairwise interchange improvement
procedure. If a better result is obtained, the new partial solution is
retained as the current partial sequence. Computational results
indicated that this approach outperformed RZ and WY heuristics.
More recently, Laha and Sarin [22] have presented a modification
of the FL heuristic, denoted as FL-LS. It implements the iteration of
the insertion step of the NEH heuristic by performing job inser-
tions rather than the pairwise interchanges. The authors proved
by numerical experiments that the modification significantly
improves the performance of the FL heuristic while not affecting
its computational complexity.
Ho [17] presented a sorting-based heuristic that includes an
iterated improvement scheme based on job insertions and pair-
wise interchanges. The author compared the method with the
heuristics of Rajendran and Chaudhuri [37] and Raj of Rajendran
[35]. In this case, larger instances of up to 50 20 were tested and
the proposed heuristic was shown to be superior. However, this
heuristic seems closer to local search techniques such as simu-
lated annealing or tabu search rather than to constructive
heuristics as its computational effort does not make it suitable
for large problem sizes and/or in those environments where
sequencing decisions are required in a short time [11].
Other heuristics assign a weight or index to every job and then
arrange the sequence by sorting the jobs according to the
assigned index. This idea was exploited by Wang et al. [45]. The
authors presented two heuristic approaches by choosing jobs
according to a given weight or index function and appending
them to a current partial sequence. The first one, named less idle
time rule (LIT), focuses on reducing machine idle times, while the
second one, named smallest process distance rule (SPD), focuses
on reducing both machine idle times and job waiting times. The
second approach also consists of two heuristics; one is based on
the Euclidean distance measure, while the other is based on the
linear distance. The authors did not compare their heuristics with
previous ones. Instead, they compared them against the lower
bound provided by Ahmadi and Bagchi [1]. The heuristics pro-
posed by Wang et al. [45] have a computational complexity of
O(n
2
m). The already mentioned RZ heuristic of Rajendran and
Ziegler [38] consists of two phases. The first phase involves the
generation of a seed sequence according to a priority rule similar
to the shortest weighted processing time, whereas the second
phase improves the solution by carrying out a local search based
on the sequential insertion of each job in the seed sequence at
each possible different position of the incumbent partial
sequence. The RZ heuristic has a complexity of O(n
3
m). Compar-
isons between the RZ and WY heuristics have been performed by
several researchers [9,26]. It was found that the RZ heuristic
performs better than the WY heuristic for small-sized problem
instances but the relative performance of the WY heuristic
improves with increasing number of jobs and finally it surpasses
Q.-K. Pan, R. Ruiz / Computers & Operations Research 40 (2013) 117–128 119