Understanding Variation in Your Charts. Module 5. What is the state of Statistical Control?. A stable mean over time with only random variation about that mean so that it is possible to predict how the system will behave in the near future. – a paraphrase of Shewhart’s 1931 definition.
*Benneyan et al. Statistical process control as a tool for research and
Healthcare improvement. Qual Saf Health Care 2003; 12:458-464
If your current average systolic BP among persons w/diabetes is 140 and you want to meet a goal of 75%of patients with a systolic BP of 130 or less, what happens?
And now your average blood pressure is 120, with half of your population less
than 120. If you medicated people to that extent, is it safe?
Rules based on
there is a difference
when there isn’t—
Type I Error = α risk
This curve is
there is a difference.
This curve is
there is no
isn’t a difference
where there is—
Type II Error = β risk
Statistical significance is defined here.
Note: Rules preferred by Carey and Lloyd.
From Amin, 2001
Indian Health Service, DHHS
*Western Electric/AT&T rules require only 8.
6. 4/5 point in a row in zone A or beyond.
7. 15 points in a row in zone C.
8. 8 points in a row on both sides of the centerline with one in zone C.
*Only use this test if the distribution is symmetrical.
Can you find another test met several times?