Using statistical process control to improve the quality of health care

Continuous improvement in health care and elsewhere is not a contentious issue-but the means by which this may be achieved is the subject of much debate. A key aspect of continuous improvement is the measurement, analysis, and interpretation of variation. Consider, for example, the data in table 1 which shows surgeon specific mortality rates after colorectal cancer surgery. ¹ Ranking the mortality rates or the adjusted hazard ratios, with or without statistical tests, invites the interpretation that some surgeons are better than others. Furthermore, since a hazard ratio of 1 is defined as neutral, surgeons with a hazard ratio above 1 are considered a hazard to their patients. So, by categorising the hazard ratio as either acceptable or unacceptable, the study concludes that " some surgeons perform less than optimal surgery; some are less competent technically than their colleagues... " To improve outcomes the next logical step is to stop the less competent surgeons from operating and transfer their patients to the more competent surgeons. Surprisingly, perhaps, there is another way of analysing these data using statistical process control (SPC) which leads to very different conclusions.

Table 1 Surgeon specific mortality rates following colorectal cancer surgery

HR, hazard ratio.

Surgeon	No of cases	No (%) died	Case mix adjusted HR
A	98	16 (16)	1.10
B	66	8 (12)	1.03
C	58	9 (16)	0.87
D	52	7 (13)	1.09
E	52	15 (29)	1.09
F	46	5 (11)	0.86
G	38	3 (8)	0.86
H	37	11 (30)	1.61
I	36	5 (14)	0.91
J	34	7 (21)	1.05
K	32	4 (13)	0.59
L	21	2 (10)	0.97
M	21	3 (14)	0.79

BACKGROUND TO STATISTICAL PROCESS CONTROL (SPC)

In the 1920s Walter A Shewhart, a physicist, was charged with improving the quality of telephones in Bell Laboratories, USA. His work there won him the accolade of the "father of modern quality control".. Two important observations can be made: (1) despite being produced by the same process, they show variation; and (2) the variation is controlled-it lies within certain limits. If nothing is known about the underlying process one would be justified in suggesting that the process appears to be stable. What would traditional approaches to understanding variation tell us about these...