Question

Explain why there is often no “lower control limit” (LCL) on a p-chart (or a c-chart).

Answer 1

The p-chart is a type of control chart used to monitor the proportion of nonconforming units in a sample, where nonconforming sample proportion is the proportion of noncomforming units in a sample of size n.

And the c-chart is a count type chart, used to monitor the number of conformities in a sample.

Now, both of these charts are are attribute charts, used for monitoring countable things; so as we cannot have a negative number of defects for controlling, we simply take the Lower Control Limit as 0. That means we have to control the process with least number of defectives zero.