Cataract Surgery Performance Monitoring Via eCUSUM

  1. Introduction

  2. Participation

  3. Procedures for CUSUM charting

  4. Outcome measurements (CUSUM parameters)

  5. Standard of performance

  6. Exclusion Criteria /Filter

  7. Pre-requisite of eCUSUM

  8. Use of CUSUM

  9. Instruction

  10. How to interpret CUSUM charts?

  11. Example of CUSUM charts

  12. Making CUSUM monitoring meaningful



Cataract surgery is the commonest procedure done in ophthalmology departments. The procedure is quite consistent and outcome is measured by visual acuity. Cataract surgery outcome depends greatly on surgeons’ skill. With advancement in technology and intraocular lens implantation, good visual outcome is almost certain among patients without pre-existing ocular co-morbidity. Hence, monitoring and evaluating competency of surgeons, especially trainees’ performance are essential and in fact mandatory in ensuring patient safety and standard of care.

Cumulative Sum (CUSUM) analysis is a statistical process control tool designed to monitor outcome of a sequential procedure performed over time by an individual operator. The CUSUM formula is as follow:
CUSUM C = max (0, Cn-1 + Xn – k)
C = case
n = no. of procedures (in a chronological, consecutive order)
Xn =outcome measure for the nth procedure.
For binary outcome, Xn= 0 (success ), Xn=1(failure)
For outcome with continuous variable, Xn=SD unit
k= reference or target value (pre-specified standard of performance, which is defined in terms of acceptable failure rate (p1) and unacceptable failure rate (p2).

The performance is acceptable as long as the CUSUM score remains below a limit or standard (h). h denotes the decision interval, determined by specifying the in-control (IC) and out-of control (OC) average run length (ARL) of a CUSUM chart.
(Refer publications on statistical analysis on CUSUM at

CUSUM analysis is displayed as a graph with numbering of sequential event at X axis and CUSUM score at Y axis. At acceptable performance when the score is below h, the chart stays flat. At unacceptable performance, when the score is above h, the chart slopes upwards and eventually crosses the decision interval if it has successive failures. When the operator overcomes his learning curve, the chart flattens to the decision interval.

CUSUM analysis has been applied in the monitoring of cataract surgery performed by trainees in Hospital Selayang since 2007. With supervisors’ intervention to trainees with unacceptable performance, we have observed dynamic positive changes in trainees’ performance, displayed as flattening of CUSUM curve. Thus, we are convinced that CUSUM technique is an objective, transparent, effective, and a robust way in monitoring surgical performance. It can be used by all cataract surgeons as self audit and as competency monitoring tool for trainees under supervision.

Cataract Surgery Registry (CSR), established since 2002, captures all the data required for CUSUM charting. With data mining from CSR, CUSUM charts can now be generated on eCUSUM web page, without needing to enter any data. By doing so, monitoring surgeons’ competency in cataract surgery is made most effective and easy.

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All cataract surgeons are welcomed to participate in eCUSUM. Those who wish to take part have to participate in the Cataract Surgery Registry. (See detail)

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Procedures for CUSUM charting


Phacoemulsification (phaco)


Extracapsular cataract extraction (ECCE)

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Outcome measurements (CUSUM parameters)


Posterior capsular rupture (PCR)


Postoperative refracted visual acuity by 12 weeks post-op, excluding patients with pre-existing ocular co-morbidity


Surgical-induced astigmatism

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Standard of performance

Standards are based on national average from the CSR findings.



Acceptable performance (P1)

Unacceptable performance (P2)


Rate of PCR for both ECCE and Phaco






Post-operative refracted visual acuity 6/12 or better by 12 weeks post-operatively






- Phaco






Surgical-induced astigmatism recorded in refraction by 12 weeks post-operatively




< 3 diopters cylinder power

> 5 diopters cylinder power

- Phaco

< one diopter cylinder power

> 2 diopters cylinder power


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Exclusion Criteria /Filter

Post-operative refracted visual outcome charting – exclude patients with pre-existing ocular co-morbidity
PCR and surgically induced astigmatism –no exclusion criteria

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Pre-requisite of eCUSUM

  1. Ensure complete records of patients who had cataract surgery are entered into cataract surgery registry (CSR) i.e. pre-clerking, operative records particularly intra-operative complications and post-operative refracted vision. For patients who do not have post-operative refracted vision but their unaided or pin hole vision is 6/12 or better, you can put this vision in the refracted vision column.

  2. As CUSUM monitoring is on specific individual surgeon, there can only be one surgeon‘s name entered in the operative record in CSR web application. The main surgeon‘s name should be entered. If both contributed to the surgery equally, the one who has created the intra-operative complication, if any, should be entered as the surgeon.

  3. When entering data to CSR web application, surgeons’ names are to be selected from the pull down list. No free text entry of names is allowed to avoid spelling mistake or same surgeon with different spelling of names.

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Use of CUSUM

  1. All surgeons
    Surgeons whose patients they operated on have records in the CSR can view his/ her CUSUM chart for self monitoring.

  2. Trainee under supervision
    CUSUM charts of trainees shall be monitored by their supervisors and head of departments. Trainees include doctors in the Postgraduate Ophthalmology training, service medical officers and gazetting specialists. Supervisors will see charts belonging to trainees assigned to them. Heads of departments will be able to review charts belonging to all the trainees and gazetting specialists.

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Steps in eCUSUM charting and viewing


Function/ Icon on eCUSUM



Log on

Go to NED website and click on to eCUSUM
Or go straight to
Type your username (same as NED web application username) and password (default as cusum).
Change your password after the first log-in


ID and Select Procedure

Ensure you are the surgeon.
Select the procedures, either ECCE or phaco.


Preview Data

Check if data are correct and complete, e.g. check that all the procedures are performed by you and data are complete. If there are missing outcome data, do retrieve patients’ note and update data in CSR web application.


Select Outcome

Select either PCR or post- op (12 weeks) visual acuity or astigmatism


Select Standard

As default



For post-op vision, do filter out patients with pre-existing ocular co-morbidity
For PCR and surgically induced astigmatism, select all subgroups
Select date. If no date is selected, CUSUM will chart all the procedures performed. The later is preferred to view the overall trend.



Click Chart and user will be able to see his/her CUSUM chart and patient’s data below the chart.
Users can save their charts by
1. Click on ‘Select a format’ - Excel or PDF format [Excel is preferred to enable data review using sort and search functions]
2. Then click ‘ Export’
3. Save in individual’s folder. Trainees can send these documents to supervisors as well.


Show data

These are data on procedures being charted.


Show Excluded Data

These are procedures which are being excluded, mainly those with missing values.
With this list, users can go to CSR web application to update the missing values.


Submit Chart

This icon will only appear on trainees log on. By clicking on submit, an auto-generated email will be sent to the supervisors and the trainees’ charts will be seen on the supervisors log-on.


Review Trainees’ Chart

This icon will only appear on supervisor’s log- on. Supervisor will see charts belonging to trainees assigned to her/him. Heads of departments will be able to review charts belonging to all the trainees and gazetting specialists.


Other instruction

When charting, user has to click one step after another in a forward direction without skipping a step. When returning to other parameters, user can go straight to step 3 : select outcome.

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How to interpret CUSUM charts?

Each event is represented as a dot on the chart. Surgeons with surgical performance within the acceptable standard will have their charts displayed along horizontal lines with no up-peak or with peaks which are between 2 decision intervals (horizontal lines drawn across the chart).

Surgeons with surgical performance outside the acceptable standard will have charts which cross the decision intervals. Once the chart has crossed a decision interval, the chart will not return to X axis. If there is no more failure, the chart slopes downward and flattens at the previous decision interval. When the chart levels off, that means the surgeon has improved his skill and has overcome the learning curve.

However, if the chart goes upward and crosses a few decision intervals, the surgeons should seek advice on ways to improve their surgical performance.

Key Points in interpretation 

Chart crosses horizontal lines when the standard of performance is outside the defined standard.

Important to see the pattern:
1. Leveling of chart after an initial upward trend indicates the surgeon has overcome the learning curve
2. Continuous upward trend crossing many decision intervals indicate poor performance needing intervention.
3. Surgeon should review detail of patients for each unsuccessful performance to determine the reasons for failure and carry out corrective or remedial action.

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Example of CUSUM charts 

1. Posterior capsular rupture (PCR)

This chart is initially flat as there is no PCR. With 3 occurrences of PCR, the chart crossed one decision interval. Following that there are a total of 6 PCR but occurring intermittently and the chart stays within 2 decision interval, denotes acceptable performance during this period.










2. Post-operative refracted vision among patient without pre-existing ocular co-morbidity

This chart reflects a typical learning curve, with initial series of unsuccessful events follows by flattening or leveling of curve. It is thus important to see the pattern of chart, i.e. upward slope, downward slope or leveling of slopes.

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Making CUSUM monitoring meaningful

  1. What is the role of trainees?
    Trainees should review their own charts, ensure data is accurate and complete, and submit the chat to his/her supervisor at a regular interval, at least once in 3 months.
    Trainees should perform an adequate number of cases to have meaningful CUSUM charts.

  2. What is the role of supervisors?
    Supervisors must review trainees’ chart once they receive the email and discuss in person during the 3-monthly progress interview. Advice and guidance should be given to trainees with poor performance. For trainees with consistently poor performance, supervisors need to supervise them closely and request for more frequent CUSUM chart submission.

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