Statistical Process Control on Pig Farms
To make an assessment of events on a farm, it is possible to use statistical process control.
For example, post-weaning mortality over time.
Mean/centre line – green line. Upper control limit – red line. Midpoint at 4.5% post-weaning mortality
In the example above
Mean calculated to be 3.0
The standard deviation is 1.3. Therefore upper control limit is 3+(3*1.3) = 6.9. Lower is 3-(3*1.3) = -0.9
The midline between upper control limit and mean is ((6.9-3)/2)+3 = 4.95
There are three rules to determine if the system process is “out of control”.
1. A single point outside control limits
2. Three out of four
consecutive points closer to the control limit than to the centre line
3. Eight or more successive points on one side of the centre line
Using the example above the system was out of control at four incidences
A = Eight or more successive points on one side of the centre line
B = Eight or more successive points on one side of the centre line – even through below
C = Three out of four consecutive points above the midline
D = A single point outside control limits – more than 3sd away from the mean
The other fluctuations are within “normal” variation.
To explain the pattern
Normal variation is explained by “Common causes” – associated with inherent random variation
Variation outside the control limits is “Special causes” – sporadic, unstable and unpredictable – a tornado or absence of personnel for example.