6.1.2. What are Process Control Techniques?

The underlying concept of statistical process control is based on a
comparison of what is happening today with what happened previously.
We take a snapshot of how the process typically performs or build a model
of how we think the process will perform and calculate control limits for
the expected measurements of the output of the process. Then we collect
data from the process and compare the data to the control limits. The majority
of measurements should fall within the control limits. Measurements that
fall outside the control limits are examined to see if they belong to the
same population as our initial snapshot or model. Stated differently, we
use historical data to compute the initial control limits. Then the data
are compared against these initial limits. Points that fall outside of
the limits are investigated and, perhaps, some will later be discarded.
If so, the limits would be recomputed and the process repeated. This is
referred to as Phase I. Real-time process monitoring, using the limits
from the end of Phase I, is Phase II.