International Journal of Advanced Engineering Technology
E-ISSN 0976-3945
Research Article
RISK ASSESSMENT OF DISTAL UPPER EXTREMITY
DISORDERS IN SMES V/S LARGE SCALE AUTOMOTIVE
INDUSTRY USING STRAIN INDEX METHOD
L P Singh
Address for Correspondence
Department of Industrial & Production Engineering, Dr B R Ambedkar National Institute of
Technology Jalandhar (Punjab), 144011. India.
E Mail lakhi_16@yahoo.com, singhl@nitj.ac.in
ABSTRACT
Musculoskeletal disorders (MSDs) of upper extremities are associated with highly repetitive occupational
activities, especially those involving high force, extreme joint postures and exposure to vibration. Strain index
(SI) is a semi-quantitative job analysis methodology based upon principles of physiology, biomechanics and
epidemiology for exposure assessment of musculoskeletal stressors of distal upper extremity (DUE). Its purpose
is the identification of jobs that place workers at increased risk of developing disorders in the DUE. The present
study finds the DUE disorders in workers of small scale forging unit verses a small scale automated casting and
large automotive industry of northern India. The average stain Index was found very high (avg=15.35, SD=7.74)
in forging unit as compare to automated casting unit and automotive car assembly plant. The workers of small
scale units are working > 8 hrs (average 10 hrs/ day). The study concluded that worker of small scale forging
units are more prone DUE disorders as compare to an automated casting unit and large automotive industry. It
also emphasized on introducing job rotations and low cost automation for material handling devices for higher
SI.
KEY WORDS: DUE disorders in SMEs, Upper extremities musculoskeletal disorders risk in SMEs.
1. INTRODUCTION
upper extremities i.e. extreme joint force with
The small scale industry (SSI) of the nation is
odd posture. Distal upper extremity (DUE) is
contributing a lot for the growth of national
defined as the elbow, forearm, wrist, and hand.
economy. Most of the small scale units are
Distal upper extremity (DUE) disorders are
involved either in exports or supplying to the
significant occupational health problems like;
large scale manufacturers. In the present
medial and lateral epicondylitis (tennis elbow),
scenario the multinational companies are
tendon entrapment at the dorsal wrist and
providing better work conditions, sanitation,
digits,
and
and
syndrome (hand). There is big lack of
emphasizing on better work postures to reduce
assessment of job risk factors of such disorders
the musculoskeletal disorders (MSDs). At the
in
same time the workers of small scale
biomechanical, or psychophysical models. The
manufacturing industry are exposed to various
objective of this paper is to briefly discuss and
hazards like noise, temperature, dust and risk
compare the levels of DUE disorders (using
of MSD. The small scale forging units
the Strain Index method) of workers deputed to
involves various kinds of high repetitive
various processes in small scale forging units
processes like hot forging, blanking punching,
and semi automated casting units.
trimming, machining (like: broaching, grinding
The Strain Index (SI) was developed by Moore
etc.). The Occupation with such high repetitive
and Garg [1] and It is primarily a method to
activities involves musculoskeletal disorders of
“analyze jobs for risk of distal upper extremity
occupational
health
facilities
IJAET/Vol.I/ Issue III/Oct.-Dec.,2010/347-357
SSI
peritendinitis,
using
and
practical
carpal
tunnel
physiological,
International Journal of Advanced Engineering Technology
E-ISSN 0976-3945
Disorders”. There are six task variable used to
2. Duration of Exertion: is the proportion of
determine the SI score. These variables are
the exertion cycle. The average length of
both qualitative and quantitative. Each of these
the exertion divided by the cycle time
variables attempts to quantify the amount of
multiplied by 100 gives the duration
the
percent.
physical
and
physiological
strain
experienced by the muscle-tendon units of the
3. Efforts per minute: is synonymous with
distal upper extremity due to physical activity
frequency of exertions per minute.
or the stress associated with the task. Each of
4. Hand/wrist posture: relates the anatomical
the six task variables is assigned a rating of 1
posture of the hand.
to 5 (Table 1 to 7). Three out of the following
5. Speed of work: estimates “the perceived
six task variables (Duration of exertion,
pace of the task and accounts for the
number of exertions/minute and duration of
stresses associated with dynamics of work
task per day) are quantitative.
6. Duration of task per day: is a measure of
1. Intensity of Exertion: is the force required
how much of the workday is allocated to
for a single performance of the task.
Table. 1 Intensity rating for strain index.
Rating criterion %Maximum
Borg
strength
scale
Light
<10%
<2
Somewhat hard
10-29%
4
Hard
30-49%
6
Very hard
50-79%
8
Near maximal
>79%
10
performing that task.
Perceived effort
Rating
Barely noticeable or relaxed effort.
Noticeable or definite effort.
Obvious effort; unchanged facial expression.
Substantial effort; changes facial expression.
Uses shoulder or trunk to generate force.
1
2
3
4
5
Table2: Percent duration of effort ratings for the strain index.
% Duration of effort
Rating
% Duration of effort
<10
1
50 to 65
10 to 19
1.5
65 to 79
20 to 29
2
80 to 90
30 to 39
2.5
>90
40 to 49
3
Table: 3 Efforts per minutes rating for the strain index.
Efforts/minutes
Ratings
<4
1
4 to 6
1.5
6 to 8
2
9 to 11
2.5
12 to 14
3
Table: 4 Hand/wrist rating for the strain index.
Rating
criterion
Very good
Good
Fair
Bad
Very bad
Wrist
extension
(degrees)
0-10
11 to 25
26-40
41-55
>55
Wrist flexion
(degrees)
0 to 5
6 to 15
16 to 30
31 to 50
>50
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Rating
3.5
4
4.5
5
Efforts/minutes
Ratings
15 to 17
18 to 19
20 to 24
>=24
3.5
4
4.5
5
Ulnar
deviation
(degrees)
0 to 10
11 to 15
16 to 20
21 to 25
>25
Perceived posture
Perfectly neutral
Near neutral
Non-neutral
Marked deviation
Near extreme
Ratings
1
2
3
4
5
International Journal of Advanced Engineering Technology
E-ISSN 0976-3945
Table: 5 speed of work for strain index.
Rating criterion
Perceived speed
Rating
very slow
slow
Fair
Fast
Very fast
Extremely relaxed pace
Taking one's own time
Normal speed of motion
Rushed , but able to keep up
Rushed and barely or unable to keep up
1
2
3
4
5
Table: 6 duration of task per day ratings for strain index
Duration of Task per day (hours)
Ratings
<1
1
1 to 2
2
2 to 4
3
4 to 8
4
>=8
5
Table: 7 combined rating values for strain index.
Rating Intensity of
values exertion
Duration of
exertion
1
light
<10
somewhat hard
10 to 29
2
3
hard
30 to 49
4
very hard
50 to 79
near maximal
>=80
5
J. S. Moore, and A. Garg,1995 [6] proposed a
Efforts per
min
Hand/ wrist
posture
Duration per
day(hrs)
<4
4 to 8
9 to 14
15 to 19
>=20
with the
very good
0 to 1
good
1 to 2
fair
2 to 4
bad
4 to 8
very bad
>8
same job titles), and some unique job
method to analyze jobs for risk of distal upper
titles to specific worksites such as (hook
extremity disorders and to distinguish between
fabricator and deck hand). Authors used
jobs which are associated with distal upper
different SI computation methods and resulted
extremity disorders from those which are not.
in significantly different SI scores along with
Phillip Drinkaus, Donald S. Bloswick, Richard
different risk level classifications calculated by
F. Sesek, and Clay Mann, Thomas Bernard [8]
the
explored one method of modifying the existing
Pourmahabadian M., Saraji J.N., Aghabeighi
tool, “The Strain Index”, to assess the
M. and Saddeghi-Naeen H, 2005 [7] observed
ergonomic risk of multi-task jobs in a case
more than 50% of investigated jobs are
control setting. The author concluded that the
categorized as "hazardous" with a significant
Strain Index may be modified to estimate the
difference between SI mean in hazardous and
risk to distal upper extremity ergonomic risk
safe jobs (P < 0.0001). Jones T. and Kumar S,
injury. Stephen Bao, Peregrin Spielholz,
2007 [3] Observed 0.12 to 0.86 per person per
Ninica Howard and Barbara Silverstein, 2009
year worked incidence of upper extremity
[9] multi task jobs and evaluated the strain
musculoskeletal
index for various worksites with job titles
occupation (saw-filer). Seth Vishal, Weston
including assemblers, painters, and office
Roberta Lee and Freivalds Andris, 1999 [10]
workers (which performed very different tasks
developed a cumulative trauma disorders
between the different worksites even though
(CTD) risk assessment model for predicting
IJAET/Vol.I/ Issue III/Oct.-Dec.,2010/347-357
different
computation
injury
in
the
methods.
sawmill
International Journal of Advanced Engineering Technology
E-ISSN 0976-3945
injury incidence rates. The model was found
is capable of identifying jobs with no distal
best suited for job tasks with cycle times
upper extremity morbidity as “safe “ and jobs
greater than 4 s. Stephens John-Paul, Gordon
with distal upper morbidity as “hazardous.”
A. Vos, Edward M. Stevens Jr., J. Steven
Evidence was provided for the Strain Index’s
Moore, 2006 [11] evaluated (by estimating
generalizibility and predictive validity. 2002
average and peak hand forces, duration of
[1] provided a normal value (<=7) of SI index
exertion, cycle time, and exertions per job
for reference.
cycle, calculated percent duration of exertion
2.1 Material and method
and efforts per minute, and assigned ratings for
The study was done in small scale forging unit
five of the six Strain Index task variables) the
without automation, a semi automated casting
test–retest repeatability of published data
unit and a large scale automotive car assembly
collection and rating methods of the Strain
industry in northern India. A sample of at 50
Index by analyzing 61 jobs, twice over a 5-
workers deputed at various repetitive tasks in
month period and found that the Strain Index
each small scale forging and casting units was
has good test–retest reliability. Moore J.
selected. In small scale casting and forging
Steven and Garg Arun, 1998 [5] evaluated the
units data was collected in two parts first
effectiveness
through
of
a
corporate
ergonomics
personal
interviews
and
second
program that used a participatory approach to
through video analysis whereas in large scale
solving problems related to musculoskeletal
automotive plant data was collected through
hazards. The
video analysis only.
Corporation experienced a
A comprehensive
significant decrease in the percentage of
questioner was developed for calculating the
recordable disorders related to musculoskeletal
self reported Strain Index (SI) (Appendix A).
risk factors; a marked decrease in the lost-time
Each question was asked in local language and
incidence rate; and a marked decrease in total
the worker was explained about the strain
and per capita annual workers' compensation
parameters.
costs. Ling LEI and Youxin LIANG [4]
2.1.1 SI Calculation
studied to clarify the prevalence and risk
Each task was observed and six aspects of each
factors
musculoskeletal
task were recorded and compared with
disorders (WMSDs) among foundry workers
quantitative and qualitative measures provided
and found the dominance of lower back pain
by the SI authors. Ratings correspond with
with highest rank followed by shoulder, neck,
each measure. These metrics and their
wrist and hand pain regardless of the job title.
corresponding ratings are presented in tables 1-
The authors suggested that pain occurring in
6, summarized in Table 7 and the multipliers
the lower back, neck and shoulder are the
for each rating are shown in table 8.
predominant ergonomic problems in the metal
2.1.1.1 Intensity of Exertion is an estimate of
casting industry. Garg Arun, Moore J. Steven,.
the strength required to perform the task one
Kapellusch Jay M, 2007 [2] presented three
time. There are five rating criterion used for
different studies showing that strain Index (SI)
this variable: “Light”, “Somewhat Hard”,
of
work-related
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E-ISSN 0976-3945
“Hard”, “Very Hard” and, “Near Maximal”.
In the present study case jobs at various
These criteria can be linked to percent
sections in each industry were identified.
maximal strength, the ten-point Borg Scale,
Videos of each job were recorded and
and Perceived Effort. Table 1 illustrates these
observed. The focus for videography was the
relationships.
upper extremities of the worker. Repeated
Duration of Exertion is calculated
videography was done to ensure that the full
by measuring the duration of all exertions
cycle of the worker was covered so that the
during an observation period, then dividing the
focus area was clear. Each job was divided into
measured
total
work elements or tasks, these tasks are defined
observation time and multiplying by 100. For
as sub-activities of a job that could be done
the purpose of this study the total observation
before transferring the job to next workstation.
time is the cycle time.
Usually these tasks involve the same motion
2.1.1.2
effort
duration
by
the
and intensity of efforts in the repetitive cycles.
The next step was to calculate the cycle time
2.1.1.3 Efforts per Minute are determined by
by taking three observation of full cycle at the
counting the number of exertions that occur
factory shop floor and taking the average. In
during an observation period, then dividing the
each cycle two time attributes were measured
number of exertions per cycle by the duration
with a stopwatch. The first timed attribute was
of the observation period, measured in
the total time of the cycle and second was
minutes.
effort duration. Effort duration is the time
during which the operator is exerting a
significant force for that task. This time of
2.1.1.4 Hand/Wrist Posture is an estimate of
exertion is used to determine the ‘Duration of
the hand or wrist position relative to neutral.
Effort’ SI variable.
The quantitative and qualitative markers are
2.3
presented in Table 4.
After videography at the entire line of sections,
2.1.1.5 Speed of Work is an estimate of how
the analysis on the videos was carried out by a
fast the worker is working. The rating criterion
team of 3 members, in order to diminish the
guidelines are presented in Table 5.
biasness. During the video analysis, the three
2.1.1.6 Duration of Task per Day is a
parameters Cycle time, No of efforts/minute
measure of the total time that a task is
and Duration of exertion were calculated.
performed per day. It is either measured or
Afterwards the ratings for two parameters (No
obtained from plant personnel. Duration of
of efforts/minute & Duration of Exertion) was
Task per Day is divided into five categories:
done. The three parameters Intensity of
<1hr, 1-2 hrs, 2-4 hrs, 4-8 hrs and, ≥8 hrs per
Exertion, Hand Wrist movement and Speed of
day and the ratings are presented in Table 6.
work were judged for their ratings. Since as
2.2
per the survey shift for the worker is >10 hrs,
Data collection
Analysis of video
hence the average duration of task per day was
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E-ISSN 0976-3945
taken as 10 hrs and a rating of 1.75 was given
kind of unit are shown in Appendix-B. The
for DDM. Rating for the wrong or difficult
data reveals that the automation in casting has
posture was inculcated in the Hand wrist rating
reduced SI score significantly, and thus the
itself.
risk of DUE disorders has been reduced except
2.4
Determine the multipliers
the tasks like molding and grinding of medium
Each variable’s multiplier is determined by its
components of automobile or valves. However
rating. The relationship between the SI rating
workers are having problems of back ache,
scores and the SI multipliers are illustrated in
neck stiffness but DUE disorders are found
Table 8.
less in the tasks under consideration. The job
Calculations of SI Index
of forger in hammer section was near the
On the basis of the ratings obtained during the
normal SI value (7.0) [1] in standard method as
video analysis, six multipliers were obtained in
well as in self reported subjective response.
accordance to the same. The multipliers are
The job of rope puller was found to be highly
Intensity of exertion (IEM), Duration of
strained in upper limbs with highest SI value
exertion (DEM), Efforts per minute (EMM),
both in video as well as in self reporting
Hands/wrist posture (HPM), Speed of work
response analysis. The workers performing
(SWM) and Duration per day multiplier
other tasks were also prone to high strain
(DDM). On the basis of the multipliers, the SI
except in broaching and grinding of small size
Index was calculated as:
components. The majority of the workers are
Strain Index;
found with high strain index. It is very clear
from the overall SI score that workers of small
3.1 RESULTS AND DISCUSSIONS
scale forging industry are highly exposed to
The SI score obtained through video analysis
distal upper extremities strain as compare to
of tasks and self reported SI score at different
semi automated small casting unit. It is
sections is shown in Tables 9 to 11. In forging
therefore observed that there is a dire need of
unit SI score calculated from video analysis
implementing the job rotation on highly
ranges from 6.56 to 26.25 (avg=15.35,
stained jobs to lower the possibility of
SD=7.74) where as the SI score obtained
musculoskeletal disorders of upper limbs with
through subjective responses ranges from 5.25
provision of regular rest pauses. The study
to 23. 62 (avg=14.72, SD=7.42). In casting
therefore recommends that time standards for
unit the SI ranges from 3.52 to 17.35
workers performance levels should be prepared
(avg=7.52, SD=4.62) where as the self
by incorporating the job rotation as well as the
reported SI ranges from 4.25 to 19.25
regular rest pauses. It is also recommended
(avg=8.78, SD=5.03). The average SI score of
that low cost automation and gravity chutes
automated car assembly unit is 8.5 (SD =
must be introduced for material handling at
4.72). The comparison of average SI score of
small distances.
the three plants is shown in figure1. The
As far as the self reporting SI values are
calculations of SI for each work station in each
concerned, these were found at lower side
IJAET/Vol.I/ Issue III/Oct.-Dec.,2010/347-357
International Journal of Advanced Engineering Technology
E-ISSN 0976-3945
(except hot material handling task) as compare
Although the SI values of forging unit found
to the SI values found by the video analysis,
by both methods differed by (-0.44 to 7.00),
which can be attributed to the following
even there was significant correlation (0.5547
reasons:
at 99 % confidence level) for forging section
1.
The workers subject to the study have
and (0.5305 at 99 % confidence level) for
been working at the same job for at least
machining section between the SI score
5 years and thus they have become
obtained by both the methods. There was a
habitual of working at the observed level
highest difference between average video
of intensity and speed of work. Therefore
analysis SI score and self reported SI score for
they perceive the job as less strenuous.
hot material handling (from furnace to forger)
The majority of the workers were
at forging section (Table 9). The reason for
habitual smokers and tobacco consumers,
high self reported SI score for hot material
which could be one of the reasons that
handling task could be that the workers may
the workers do not feel much strain.
feel high strained due to high ambient
The majority of the workers are illiterate
temperature and insufficient ventilation.
2.
3.
or less educated; therefore they are not
aware
of
musculoskeletal
disorders
caused by strenuous repetitive jobs.
Table:8 Multipliers for strain index
Rating
values
1
1.5
2
2.5
3
3.5
4
4.5
5
Intensity of
exertion
multiplier
(IEM)
1
2
3
4.5
6
7.5
9
11
13
Duration
of exertion
multiplier
(DEM)
0.5
0.75
1
1.25
1.5
1.75
2
2.5
3
Efforts per
minute
multiplier
(EMM)
0.5
0.75
1
1.25
1.5
1.75
2
2.5
3
Hands/wrist
posture
multiplier
(HPM)
1
1
1
1.25
1.5
1.75
2
2.5
3
Figure 1 Overall Average SI of casting, forging and car assembly line.
IJAET/Vol.I/ Issue III/Oct.-Dec.,2010/347-357
Speed of
work
multiplier
(SWM)
1
1
1
1
1
1.25
1.5
1.75
2
Duration
per day
multiplier
(DDM)
0.25
0.375
0.5
0.625
0.75
0.875
1
1.25
1.5
International Journal of Advanced Engineering Technology
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Table.9 Strain Index score associated with various processes in forging unit.
Sr. No.
Forging
Section
1)
2)
3)
Machining
Section
1)
Section
Job
Description
Rope puller
Small
Forger
Components
Hot material handler
Rope puller
Medium
Forger
Components
Hot material handler
Rope puller
Large
Forger
Components
Hot material handler
Punching
section
2)
Broaching
section
3)
Grinding
section
Large component1
Very small Components 2
Medium Components 3
Small Components 4
Small Components1
Medium Components 2
Large Components 3
Belt grinding small components1
Duplex grinding Medium
Components 2
Duplex grinding Large
components 3
Wheel grinding 4
Average (SI)
Std. Deviation (SI)
Strain Index (SI)
Video Analysis
26.25
9.84
14.76
26.25
8.20
14.76
21.00
6.56
19.68
Strain Index (SI)
Self reported
15.75
5.25
23.63
18.38
7.88
23.63
18.38
9.18
23.63
27.56
8.20
13.13
9.84
6.56
9.18
10.5
6.56
15.75
9.84
15.75
9.84
7.87
9.18
10.5
7.87
26.25
31.5
26.25
7.00
15.75
15.35
7.74
23.62
14.72
7.42
Table.10 SI Strain Index score associated with various processes in automated casting unit.
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International Journal of Advanced Engineering Technology
Sr
No.
1
2
3
4
5
6
7
8
9
10
11
12
Job Description
Cleaning and Matching of Moulding Boxes
Clamp Fitting at Moulding boxes (1)
Strain Index (SI)
Video Analysis
3.52
4.69
Clamp Fitting at Moulding boxes(2)
Large M/C Moulding man(1)
Large M/C Moulding man(2)
M/C molder (Medium box) machine control
M/C molder (Medium box) with shoveling
M/C molder (small box) machine control
M/C molder (small box) with shoveling
Manual Moulding
Grinding (small components)
Grinding (medium Components)
Average (SI)
Std. Deviation (SI)
4.10
3.94
4.50
7.20
13.12
4.50
6.56
7.03
13.75
17.35
7.52
4.62
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Strain Index (SI) Self
reported
4.25
5.60
5.20
3.65
4.56
8.10
15.90
6.89
10.25
8.15
13.67
19.25
8.78
5.03
Table.11 Average SI Strain Index score associated with various processes in automated car
assembly unit.
Assembly line
Trim-1
Trim-2
Trim-3
Chassis 1
Chassis 2
Final Assembly
Grand Average
Average SI Score
7.87
8.44
6.71
10.99
8.4
8.59
SD of SI Score
2.21
4.44
4.07
6.97
5.3
5.34
8.5
4.72
4.1 CONCLUSIONS:
The following conclusions were drawn from
the study:
1. The highly strained jobs should be rotated
after every 2-4 hours with the low SI score
jobs.
2. For the non rotatable jobs regular rest
pauses must be provided for job at various
sections of forging units.
3. The automation in small scale casting unit
has resulted with lower SI score.
4. The low cost automation should be
implemented for material handling and
lifting.
5. Performance
standards
should
be
calculated by incorporating rest pauses to
minimize the rate of DUE disorders.
5.1 ACKNOWLEDGEMENT
The authors acknowledge the co-operation and
help extended by the management and the
workers of small scale (casting &forging units)
and large scale manufacturing plant, for
conducting personal interviews, making
observations and videography at each section
very thoroughly.
Appendix-A
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E-ISSN 0976-3945
Questionnaire for Interview.
Q.1 How much effort do you exert during work?
a) Light,
b) Somewhat hard, c) Hard,
d) Very hard,
e) Extreme
Q.2 Do you get adequate rest pauses during work cycles?
a) Maximum, b) More than adequate, c) Adequate, d) Least,
e)Not at all
Q.3 What is the frequency of efforts exerted (per minutes) by your
upper limbs?
a) Least, b) Moderate, c)Tolerable, d) More than tolerable, d)
Maximum
Q.4 What is the posture of your upper limbs during work?
a) Very good,
b) Good,
c) Fair,
d) Bad,
e) Very bad
Q.5 At what pace you are required to perform your task?
a) Very slow,
b) Slow,
c) Fair,
d)Fast,
e) Very fast
Q.6.What is working time /day?
a) Up to 8 hours, b) to 10 hours, c) >10 hours
Q.7 Do you tobacco / smoke at work place?
a) Yes
b) No
(Signature of Worker)
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3.
4.
Ergonomics Today @ ergoweb August 20,
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Garg Arun, Moore J. Steven, and Kapellusch
Jay M, “The Strain Index to Analyze Jobs for
Risk of Distal Upper Extremity Disorders:
Model Validation”; Proceedings of the 2007
IEEE IEEM.
Jones T. and Kumar S, 2007, Comparison of
ergonomic risk assessments in a repetitive
high-risk sawmill occupation: Saw-filer,
International
Journal
of
Industrial
Ergonomics, 37, 744–753.
Ling LEI and Youxin LIANG, “Prevalence
and risk factors of musculoskeletal disorders
of foundry workers”; Department of
Occupational Health, FUDAN University
School of Public Health 138 Yixueyuan Rd.,
Shanghai, China.
IJAET/Vol.I/ Issue III/Oct.-Dec.,2010/347-357
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