Abstract
Aim
To apply the control chart method to retrospectively analyse the variation in postcataract surgery endophthalmitis.
Methods
The endophthalmitis data between 01/07/97 and 30/06/07 (10 years) at the Royal Gwent Hospital were analysed using the control chart method. Certain criteria related to the control chart were employed to detect unusual variations and early outbreaks of endophthalmitis.
Results
A total of 21 032 cases of cataract were performed in that period, with a mean annual incidence of 0.16%. The control chart could provide a reliable method to detect an early outbreak of endophthalmitis before it was suspected and acted on. The chart provided visual information of the altering trends of endophthalmitis cases and allows unusual variations to be detected early.
Conclusion
The control chart is potentially a sensitive method to detect anomaly in the cases of postcataract surgery endophthalmitis. Its control limits are dynamic and factor in recent performance to detect any unusual variation.
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Appendix
Appendix
Calculation for the lower control limit from point 1–17 (taken at 2 standard deviations)
The Moving Range (MR) mean needs to be calculated to obtain the Standard Deviation (SD). The MR is defined as the difference between the number of non-infected cases between two subsequent cases of endophthalmitis, for example, the MR between cases 1 and 2 (see Figure 1) is 807−618=189.
The mean MR for first 17 consecutive cases of endophthalmitis, y=(189+208+118+77+634+556+39+158+37+20+80+701+751+41+78+647)/16=270.875
Mean of non-infected cases between endophthalmitis cases from cases 1–17, z=(807+618+410+528+451+1085+529+490+332+369+389+469+1170+419+378+456+1103)/17=588.412
For this method of control chart calculation at 3 SD control limit, a constant A=2.659 is used (taken from the table of constant provided by Kelley 15).
SD=constant A × y/3=2.659 × 270.88/3=240.090
Therefore, the control limit at 2 SD
=z−2SD
=588.412−2(240.090)
=108.2
Calculation for the lower control limit from points 27–34 (taken at 2 SD)
The mean MR from points 27–34, a=(130+399+733+413+364+422+452)/7=416.413
Mean of non-infected cases between endophthalmitis cases from cases 27–34, b=(527+657+1056+1689+1276+912+1334+1786)/8=1154.625
SD=constant A × a/3
=2.659 × 416.413/3
=369.081
Therefore, the control limit from points 27–34
=b−2SD
=1154.625−2 (369.081)
=416.5
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Chiam, P., Feyi-Waboso, A. The use of control charts in monitoring postcataract surgery endophthalmitis. Eye 23, 1028–1031 (2009). https://doi.org/10.1038/eye.2008.257
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DOI: https://doi.org/10.1038/eye.2008.257
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