REFEREE 3 ---------------------------------------------------------------------------------------------------------------- Major Compulsory Revisions
COMMENT 1: The authors should be complimented for having conducted an interesting study
developing our knowledge on exposure assessment techniques. It appears that
the manuscript is a revised version of the initial submission. However, it is
impossible to know if the authors have addressed issues raised by the referees
(no attached point to point answers).
RESPONSE 1: We apologize for the reviewer’s confusion. This was an initial submission, not a revision,
and therefore we did not have the requested documentation. For this resubmission, we have included a
point by point description of our responses.
COMMENT 2: The aim of this study was to evaluate predictions of the median and range of
amplitude of trapezius muscle activity and the median and range of motion of
shoulder, head, neck, and torso postures based on measurements collected
during computer use in a real-life work setting. The authors fulfill in general this
aim adequately. However, one can question from time to time the choice of the
assumption made by the authors (see below) and of some of the wording (e.g.
“comprehensive”). In general, the results could somehow be expected taking the
number of sets of parameters used for “task-based” and “comprehensive”
predictions.
RESPONSE 2: With respect to the wording, we have replaced the word “comprehensive” with “fullmodel” throughout the manuscript. We especially thank the reviewer for this comment, as now our
language is more consistent with our previous publication (Bruno Garza et al 2012).
With respect to the comment that the “results could be somehow expected,” we still feel that they are
interesting and useful to report. We now explicitly draw the conclusion that:
“Based on these results, we can conclude that the variation in muscle activities and postures across
users performing the same computer task is large, especially in comparison to the variation across
tasks.”
To further elucidate the utility of our results. We feel that our results have important methodological
and clinical implications, which we now explicitly state:
“Methodologically, this finding supports the use of full models including a wide range of task,
questionnaire, workstation, and anthropometric parameters to predict physical exposures during
computer use instead of the task-based predictions to account for this variation. Clinically, this finding
supports the idea that computer users will have differences in their muscle activity and postural
exposures even when performing the same tasks, possibly putting these users at different risks for
developing musculoskeletal symptoms.”
COMMENT 3: Abstract: The authors should envisage replacing “comprehensive” by another word.
RESPONSE 3: Please see response 2 above.
COMMENT 4: Introduction: Rephrase the first sentence - as such the link between computer work and
MSD is not clearly established see e.g. review by Waersted et al (2010), Ijmker et al
(2011) and Andersen et al (2008)
RESPONSE 4: We have rephrased the sentence to read:
“Because of the high incidence and prevalence of musculoskeletal disorders and their symptoms that
has been noted among computer users, computer use is believed to be an important determinant of
musculoskeletal disorders and their symptoms (IJmker et al. 2007, Eltayeb et al. 2007, Wahlstrom 2005,
Gerr et al. 2002).”
Please also note the additional references that we have added to support this statement (Eltayeb et al.
2007, Wahlstrom 2005).
COMMENT 5: In the first paragraph, it can seem peculiar that the authors did not consider the
duration of physical exposure as important? While the variability term is may be
overrepresented.
RESPONSE 5: We have included the following in the discussion to address this comment:
“Both increases and reduction in ranges of muscle activities and postures may cause damage to
musculoskeletal tissues (Srinivasen and Mathiassen 2012, Visser and van Dieen 2006, Barbe and Barr
2006). Other exposures and exposure metrics, such as duration or frequency of exposure, may also be
related to the development of musculoskeletal symptoms, and the viability of predictions of these for
exposure assessment could be considered in future studies. However, we should note that duration can
already easily be measured with computer interaction monitoring software that is commercially
available and that has been used in recent epidemiological studies (Chang et al. 2008, IJmker et al.
2011).”
COMMENT 6: In the second paragraph, “limited success”, please explain
RESPONSE 6: We have expanded on this statement in this paragraph:
“In the field of computer work, Bruno Garza et al. (2012a) used both task and anthropometric
parameters to predict muscle activities and postures during computer use, with limited success. Their
predictions of muscle activities, especially, corresponded to very low R2 values and high RMS and
relative RMS errors.”
COMMENT 7: Methods: The ethical approval seems strange – Can the Harvard school/ VU ethics
committees approve study conducted in NL/USA? Give approval number.
RESPONSE 7: We now include the following description of our ethical approval:
“This project was approved by the applicable Institutional Review Boards for protection of human
subjects (Harvard School of Public Health Office of Regulatory Affairs and Research Compliance, protocol
#17938-105, The Medical Ethics Committee Independent Review Board of the VU University Medical
Center in Amsterdam, registered with the US Office of Human Research Protections as protocol
#IRB0002991).”
COMMENT 8: “lower biceps brachii” ??
RESPONSE 8: We have included the phrase “on the distal part of the upper arm, above the elbow
crease”, to clarify the marker location.
COMMENT 9: Please elaborate the following assumption: “assumption that exposures within a
task (keyboard, mouse, and idle) do not vary largely across individuals”
RESPONSE 9: We have changed the language to mirror our description of this assumption in the
introduction: “Hence, task based estimates were calculated based on the assumption that there is large
variation in muscle activities and postures across tasks and small variation in exposures within tasks.”
COMMENT 10: Torso-posture: it seems peculiar to have this as a predictor for neck-shoulder or
upper extremities WMSD
RESPONSE 10: We believe that torso posture, as well as the other muscle activities and postures, are all
risk factors for the development of musculoskeletal disorders during computer use that have been
identified in previous studies (note the additional references that we have included to these studies).
Biomechanically, the torso provides the base for both the shoulder and the neck and for this reason we
also believe that the torso may be important. We have added the following paragraph to the discussion
to clarify this hypothesis:
We chose to study both the median and range of trapezius muscle activity and shoulder, head,
neck, and torso postures because we believe that each of these exposures may be relevant to
the development of musculoskeletal symptoms. All muscle activities and postures selected for
this study have been identified as potential risk factors for musculoskeletal symptoms during
computer use in previous studies (e.g. Starr et al. 1985, Marcus et al. 2002, IJmker et al. 2008)
COMMENT 11: 103 parameters. Refer to the Appendix, why 103?
RESPONSE 11: First, please note that we had a mistake in the previous version of the manuscript, in
which we incorrectly stated that there were 103 parameters instead of the actual 104 parameters that
we did use in our study. We have made the correction and now describe throughout the manuscript
that there are 104 parameters.
In response to the comment regarding why we chose 10(4) parameters, this choice was based on our
effort to include all parameters that we expected, based on previous studies, could be related to our
physical exposures during computer use. We have included the following information in the discussion,
along with references to these previous studies:
“In this study, we strove to include all parameters that we expected, based on previous studies, could be
related to our physical exposures during computer use (e.g. Bruno Garza et al. 2012b, Bruno Garza et al.
in review, IJmker et al. 2008, Marcus et al. 2002, Won et al. 2009). It is possible that there may be other
parameters than the 104 described here that we or others have not yet considered or explored.”
COMMENT 12: Please elaborate also the following assumption: “unbiased measurements”
variables?
RESPONSE 12: We rewrote the sentence to read:
“Thus, we assumed that there is no systematic misclassification bias.”
COMMENT 13: Results: “There was a large range of parameters used for the comprehensive predictions
(Table 5).” Compared to what? Unclear?
RESPONSE 13: We have reworded the sentence to read:
“There was a large range of parameters included in the full-model predictions across the seven
categories.”
COMMENT 14: “one of the three task”?
RESPONSE 14: We have removed this language from the manuscript. The sentence now reads:
“The “percentage keyboard use” (#46) or “percentage mouse use” (#47), task parameters were used to
predict three of the fourteen physical exposures.”
COMMENT 15: Discussion: Please repeat briefly what the audience needs to understand behind the
terms “task-based approach” and “comprehensive approach”
RESPONSE 15: We have reworded the first paragraph of the discussion to make the difference between
the task based and full-model approaches more clear:
“The full-model predictions (based on a comprehensive set of task, questionnaire, workstation, and
anthropometric parameters) out-performed task-based predictions (based on the distribution of
keyboard/mouse/idle activities only)”
COMMENT 16:“we strove to include all measurable…” many more parameters can be included.
A statement mentioning why 103 parameters were selected would help
RESPONSE 16: Please see response 11.
COMMENT 17: “One explanation for this unexplained…” rephrase
RESPONSE 17: We have reworded the sentence:
“Another possible reason for this unexplained variation could be that the computer workstation
environment affords different strategies for interacting with the computer, allowing participants to
perform the same tasks using different postural strategies in a way that can be difficult to capture using
measureable variables (Mark et al. 1997).”
COMMENT 18: “…workstation affords different strategies for interacting with the computer…”?
RESPONSE 18: We have reworded the sentence for clarification:
“Another possible reason for this unexplained variation could be that the computer workstation
environment affords different strategies for interacting with the computer, allowing participants to
perform the same tasks using different postural strategies in a way that can be difficult to capture using
measureable variables (Mark et al. 1997).”
COMMENT 19: The authors should specify a little more why they decided to elaborate on their
previous work
RESPONSE 19: We have elaborated on this choice in the discussion:
“The similarities and differences in the findings of this study compared to a previous laboratory study
(Bruno Garza et al. 2012a) can inform future prediction methods. Bruno Garza et al. (2012a) reported
low R2 values and large RMS errors for trapezius muscle activity using predictions based only on task and
four variables describing individual factors: age, gender, body mass index, and shoulder width. In the
current field study, much better predictions of trapezius muscle activity were produced using mostly
parameters from categories other than individual factors. While the quality of predictions of shoulder
postures was not very different between the laboratory study and the current study, in this study we
demonstrated that these physical exposures can still be predicted as well in the field, where there are
likely more sources of variability to affect physical exposures, as in the laboratory. Finally, the current
study provided predictions of head, neck, and torso postures, which were not calculated in the
laboratory study.“
COMMENT 20: Even though, this is not within the scope of the paper, the audience will expect
an explanation (does it play a role, is this good or bad?) (“no previous study has
demonstrated the association between range of amplitude/range of motion and
the use of multiple computers simultaneously.”)
RESPONSE 20: We have included the following sentence in the discussion:
“However, this association seems plausible, since it is likely that a computer worker who uses multiple
computers would have to move around more within the workstation, increasing the ranges of amplitude
and motion and possibly decreasing the risk for developing musculoskeletal symptoms.”
However, please note that we are hesitant to state this type of clinical conclusion too strongly, because,
as we discuss in the “limitations”:
“The aim of the study was not to identify causal relations between specific parameters and either
muscle activity or postures. The parameters may be confounded by each other, or may be correlated
with one another. Also, our parameter selection procedure required multiple testing, so it is possible
that the parameters were selected based on chance.”
COMMENT 21: The statement made on workstation adjustability can be misunderstood, please
rephrase (“adjustability of workstation chairs did not have an effect on neck and
upper limb postures”)
RESPONSE 21: We have reworded the sentence:
For instance, Gerr et al. (2000) reported that adjustability of workstation chairs did not change neck or
upper limb postures, while we used chair height to predict median left shoulder abduction and neck
flexion.”
COMMENT 22: “An important test of our predictions, which we were unable to perform in the
current study but should be explored in the future, is whether they are able to
predict health outcomes.” rephrase see comments on computer usage and
WMSD
RESPONSE 22: As noted in response to comment 4, the associations between biomechanical exposures
and MSDs are not fully established. We suggest that the approach we have taken to exposure
assessment may contribute to ongoing work examining the relationships between biomechanical
exposure and development of MSDs.
COMMENT 23: Add the common limitations related to the use of questionnaire (overestimation,
self-bias selection, health concern) as well as critics concerning the leisure time
activities
RESPONSE 23: We have added the requested information to the discussion:
“Any interpretation of the individual parameters used to calculate the full-model predictions must be
done with caution. Previous studies have shown that questionnaire responses can be biased, possibly
due to overestimation, self-bias selection, health concerns, etc., leading to misclassification of our
parameters (IJmker et al. 2008). . The parameters may also be confounded by each other, or may be
correlated with one another. Additionally, our parameter selection procedure required multiple testing,
so it is possible that the parameters were selected based on chance. However, the aim of the study was
not to identify causal relations between specific parameters and either muscle activity or postures.”
COMMENT 24: The “limited variability” was partly remediated by the use of min/max values that
may have actually result in poorer prediction (inclusion of outliers)
RESPONSE 24: We did not, in fact, use the min/max values for the parameters, but actually used the
participant’s actual responses. Our interpretation of “limited variability” was based on the small
standard deviations for some of the parameters. We have reworded the sentence for clarification:
“Second, we had limited variability within the categories of some of our parameters (as demonstrated
by the small standard deviations in Appendix A), which may have limited the utility of these parameters
for predicting our physical exposures.”
REFEREE 1 ----------------------------------------------------------------------------------------------------------------------Reviewer's report
COMMENT 1: This study attempted to predict the muscle activities of the trapezius and posture
change during real-life computer work for approximately two hours at the
subjects' own workstations. The authors used two kinds of methods for
predictions, i.e. the task based prediction (based on the duration of the
keyboard/mouse/idle tasks) and the comprehensive prediction (based on the
questionnaire, workstation, and anthropometric parameters). The manuscript is
generally well written, while the clinical relevance is not adequately revealed.
RESPONSE 1: We should have stated that our main research question and hypothesis addressed a
methodological issue, not a clinical issue. However, thanks to your suggestion, we have updated the
manuscript to address the clinical relevance of our study:
ABSTRACT CONCLUSION: “There is more variation in physical exposures than what can be explained by
task. Thus, a comprehensive set of parameters will provide more accurate predictions for exposure
assessment of muscle activities and postures during computer use for future epidemiological studies.”
DISCUSSION: “Clinically, this finding supports the idea that computer users will have differences in their
muscle activity and postural exposures even when performing the same tasks, possibly putting these
users at different risks for developing musculoskeletal symptoms.”
Other concerns are as follows:
COMMENT 2: Title: The title is suggested to be shortened such that it will be easy to understand
for the readers. For example: Prediction of trapezius muscle activity and shoulder, head, neck,
and torso postures during computer use: results of a field study.
RESPONSE 2: We have changed the title as suggested.
COMMENT 3: Abstract: The calculation of the parameters is not necessary tediously described (ex.
RMS errors: the square root of the squared observed minus predicted values
averaged over all participants)
RESPONSE 3: Due to the limited space requirements in the abstract, we have removed the description of
the calculation of RMS and relative RMS from the abstract and will instead allow readers to refer to the
methods section, in which we have a more detailed description of the calculation of these parameters.
COMMENT 4: Abstract Results: "RMS errors (relative RMS errors)... (for posture) (9-19%)." The
sentence is not easy to read.
RESPONSE 4: We have reworded our abstract results to make them clearer and easier to read:
“The full-model predictions of the median and range of amplitude of trapezius muscle activity had
consistently better R2 values (range 0.43-0.57 compared to.00-0.06), RMS errors (range 2-3%MVC
compared to 3-4%MVC), and relative RMS errors (range 10-14%MVC compared to 16-19%MVC) than the
task-based predictions. The full-model predictions of the median and range of amplitude of postures
also had consistently better R2 values (range 0.16-0.59 compared to 0.00-0.35), RMS errors (range 2-12
degrees compared to 3-22 degrees), and relative RMS errors (range 9-19 degrees compared to 13-42
degrees) than the task-based predictions.”
COMMENT 5: Abstract Conclusion: No clinical relevance is provided.
RESPONSE 5: We have added the requested information, please see response 1.
COMMENT 6: [Introduction] The description "the median and range of amplitude of trapezius muscle
activity and the median and range of motion of shoulder, head, neck, and torso
postures" is repeatedly used in the whole article. It is suggested to use an
alternative term to substitute the sentence afterwards.
RESPONSE 6: Throughout the manuscript, we have substituted the phrase “physical exposures” for the
longer description. We have left in the more specific language where we feel that it is necessary in order
to provide clarification of our intentions.
COMMENT 7: The goal itself focuses on the methodology. There is no rationale about the
importance to predict the muscle activity of the trapezius (more muscles involved
in the computer work) and the shoulder/head/neck/torso motions (upper
extremity movements could also lead to work-related disorders).
RESPONSE 7: Please see response 1. With respect to the rationale for why we chose our muscle activities
and postures of interest, we have added the following paragraph to the discussion:
“We chose to study both the median and range of trapezius muscle activity and shoulder, head, neck,
and torso postures because we believe that each of these exposures may be relevant to the
development of musculoskeletal symptoms. All muscle activities and postures selected for this study
have been identified as potential risk factors for musculoskeletal symptoms during computer use in
previous studies (e.g. Starr et al. 1985, Marcus et al. 2002, IJmker et al. 2008) Both increases and
reduction in ranges of muscle activities and postures may cause damage to musculoskeletal tissues
(Srinivasen and Mathiassen 2012, Visser and van Dieen 2006, Barbe and Barr 2006). Other exposures
and exposure metrics, such as duration or frequency of exposure, may also be related to the
development of musculoskeletal symptoms, and the viability of predictions of these for exposure
assessment could be considered in future studies.”
COMMENT 8: [Method] The accuracy and reliability of the triaxial accelerometers to quantify the
shoulder/head/neck/torso motion should be provided to ensure the data quality
used in the prediction model.
RESPONSE 8: We added the statement:
“The accelerometers have a range from -/+2g (gravitational acceleration) with an accuracy of 10mg and
resolution of 1.5mg.. Inclinometers are a reliable and accurate method for measuring upper extremity
postures (Teschke et al. 2009, Svendsen et al. 2005).”
COMMENT 9: The description about the "percent increase in participants... " is not clear.
What is the "direct measurements" being compared with?
RESPONSE 9: We have reworded the sentence to clarify our intentions:
“We calculated the percent increase in participants that would be required in order for our physical
exposure predictions to have the same power to detect differences as the direct measurements.”
COMMENT 10: [Results] The authors presented all the measurements in median and range of value.
Thus there are similar tables and figures (ex. Table 2 and 4; Table 3 and 5). The
authors are suggested to ponder which kind of data is more representative and to
avoid redundancy.
RESPONSE 10: We agree with the reviewer that the median and range of values analyses do not tell
different stories. In both cases, the full-model predictions are much better than the task-based
predictions. However, since median and range are different exposure metrics that may have different
pathways to musculoskeletal disorders, it is important to predict both metrics for future epidemiological
studies. We have added the following paragraph in the discussion to address the importance of both
median and range of physical exposures as predictors of musculoskeletal disorders:
“We chose to study both the median and range of trapezius muscle activity and shoulder, head, neck,
and torso postures because we believe that each of these exposures may be relevant to the
development of musculoskeletal symptoms. All muscle activities and postures selected for this study
have been identified as potential risk factors for musculoskeletal symptoms during computer use in
previous studies (e.g. Starr et al. 1985, Marcus et al. 2002, IJmker et al. 2008) Both increases and
reduction in ranges of muscle activities and postures may cause damage to musculoskeletal tissues
(Srinivasen and Mathiassen 2012, Visser and van Dieen 2006, Barbe and Barr 2006). Other exposures
and exposure metrics, such as duration or frequency of exposure, may also be related to the
development of musculoskeletal symptoms, and the viability of predictions of these for exposure
assessment could be considered in future studies.”
COMMENT 11: How the finding "Hence, a smaller percent increase is needed for the
comprehensive predictions than for the task based predictions" derives from?
There was no comparison between the two predictions in Figure 1.
RESPONSE 11: We have reworded the sentence to clarify our findings:
“The percent increase in participants required for each median and range of amplitude of trapezius
muscle activity and each median and range of amplitude of shoulder, head, neck, and torso postures in
order to have the same power to detect differences as the direct measurements is inversely
proportional to the R2 value for that exposure. Since the R2 values were consistently larger for the fullmodel predictions, a smaller percent increase is needed for the full-model predictions than for the taskbased predictions. The percent increases for each of the median, range of amplitude, and range of
motion full-model predictions are shown in Figure 1.”
COMMENT 12: [Discussion and Conclusion] The results from the "median" and "range of value" of all
measurements were not used to show different stories. Again, the authors should condense the major
findings.
RESPONSE 12: Please see response 10.
COMMENT 13: The R-square values were generally low even for comprehensive prediction
model which could hardly persuade the readers of its merits. The residual
analysis might provide more information about how those parameters correlate
with the physical exposures.
RESPONSE 13: We feel that we have performed a residual analysis by reporting our RMS errors and
relative RMS errors along with the R-squared values. We have included more information to describe
these two parameters and their relationship to residuals in the methods section:
“Root mean squared (RMS) errors were calculated by subtracting the predicted values from the
corresponding observed values, squaring them, and then taking the square root of the averaged squared
errors. RMS errors describe the averaged absolute differences between observed and predicted values
(average residuals). Relative RMS errors were determined by dividing the RMS error by the full range of
median, range of amplitude, or range of motion values observed (maximum-minimum values shown in
Table 1). Relative RMS errors describe the average residuals while taking into account the variation in
the observed values (normalized residuals).”
COMMENT 14: There was no discussion about Figure 1 such that it only provides trivial
information.
RESPONSE 14: We have included a reference to figure 1 in the paragraph discussing these results in the
discussion to make the link more apparent:
“All parameters used for muscle activity and posture predictions were measured using a questionnaire,
computer monitoring software, or tape measure, methods which are significantly less time and cost
intensive than using direct measurements. For this reason, even though a larger sample size of workers
would be required due to the decreased statistical precision of predictions compared to direct
measurements (figure 1), we believe that these predictions may be useful for examining neck and upper
limb muscle activities and postures in future epidemiological studies of physical exposures and
musculoskeletal disorders.”
COMMENT 15: The conclusion should be clinically relevant.
RESPONSE 15: We have included a clinical conclusion:
“In conclusion, the results of this study indicate that the full-model predictions, based on task,
questionnaire, workstation, and anthropometric parameters, perform better than the task-based
predictions. The method described here may have implications as an alternative exposure assessment
technique for future epidemiological studies. Additionally, our findings support the idea that computer
users will have differences in their muscle activity and postural exposures even when performing the
same tasks, possibly putting these users at different risks for developing musculoskeletal symptoms.”
REFEREE 2 ----------------------------------------------------------------------------------------------------------Reviewer's report
COMMENT 1: This is an interesting paper, with an innovative approach trying to develop a
predictive model for WMSD related to computer use, and compare these results
with actual measurement data. The paper is well written and the rationale of the
study, the experimental procedures and the results were disccused and
interpreted appropriately. I consider this paper is acceptable for publication, with no need for any major
compulsory revisions.
RESPONSE 1: Thank you very much for your kind words and thoughtful review, we appreciate it!
Some Minor revisions may be needed:
COMMENT 2: For the actual measurement of emg data from the subjects, how long was the
data capture period?
RESPONSE 2: As described in the methods section:
“Data used for this study included computer interactions, questionnaire responses, measurements of
workstation setup and anthropometry, and continuous measurements of muscle activity of the right and
left trapezius, and shoulder, head, neck, and torso posture data from 117 office workers (33 male, 84
female) performing computer work while working for approximately two hours at their own
workstations.”
COMMENT 3: Is there any consideration for the time of day?
RESPONSE 3: Yes, we did consider time of day. To clarify, we have included the following sentence in the
“methods” section:
“The measurements were balanced so that half were taken in the morning and the rest in the
afternoon.”
COMMENT 4: What tasks are being performed in the data collection period? only text typing or a variety
of tasks? in that case, a longer period may be more appropriate.
RESPONSE 4: Since the data was collected while office workers were performing their own work, these
workers did perform a variety of tasks typical to office workers including word processing, internet
surfing, etc. We have included the following sentence in the methods section to describe the worker’s
tasks:
“All workers described themselves as “office workers”, and performed a variety of typical computer
tasks during the measurement period such as word processing, internet research, emailing, etc.”
With respect to the comment about whether our sampling period is adequate, previous studies have
shown that a two hour measurement period has been shown to capture representative measurements
of workday physical exposures during computer use. We have included this statement in the methods
section:
“A two hour time period has been shown to capture representative measurements of workday physical
exposures during computer use (Johnson et al. 2000, Asundi et al. 2012).”
COMMENT 5: There are 3 main variables R2, RMS ad relative RMS - more explanation about
why these 3 variables are selected, and what are the asscoiations between them.
There can be better explanation for the audience, especially those who are not
so familiar with the statistical approaches.
RESPONSE 5: We have included the following information when describing these variables in the
methods section:
“Three values were calculated to ascertain the quality of our predictions. R2 values were calculated by
performing a simple linear regression of the predicted and measured median, range of amplitude, and
range of motion values. R2 values describe the percent variability in the observed values explained by
the predicted values. Root mean squared (RMS) errors were calculated by subtracting the predicted
values from the corresponding observed values, squaring them, and then taking the square root of the
averaged squared errors. RMS errors describe the averaged absolute differences between observed and
predicted values (average residuals). Relative RMS errors were determined by dividing the RMS error by
the full range of median, range of amplitude, or range of motion values observed (maximum-minimum
values shown in Table 1). Relative RMS errors describe the average residuals while taking into account
the variation in the observed values (normalized residuals).”