NCPA - National Center for Policy Analysis

Plotting Quality In Health Care

February 13, 2001

Many lives could be saved if a simple mathematical technique used since the 1920s to check quality control in car manufacture is applied to adverse events or performance results of hospitals, says a British team.

  • The approach could easily have picked up a clear excess of childhood deaths after heart surgery at the Bristol Royal Infirmary in 1997, a year before problems were identified.
  • It could also have highlighted the high number of deaths among the elderly women patients of Harold Shipman, the former British doctor and now convicted serial killer.

This technique has a huge number of applications -- for hospitals, schools and universities, as well as manufacturing. But the U.K. Department of Health argues that while the technique is widely used in industry, it may be too simplistic for use with hospital data.

The graphical method was developed by U.S. physicist Walter Shewhart for use in the manufacturing industry. In medical applications, instead of placing scores in a league table -- a table of ranked statistical data -- the number of adverse outcomes is plotted against the total number of cases on a graph. A line is drawn through the mean (average), and all scores within three standard deviations -- a common measure of variation used in research -- are considered to be normal variations in the system. In practice, most of the scores will fall within three standard deviations. Any scores outside the "control limits" suggest a special cause.

In a league table, there is always a top value and bottom value, but that doesn't necessarily mean any kind of intervention should be taken. One of the key advantages of using the graphical approach is that it makes it clear when to intervene or not.

Source: Mohammed, et al., "Bristol, Shipman, and Clinical Governance: Shewhart's Forgotten Lessons," Lancet, February 10, 2001.

 

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