A p chart plots the proportion defective in each sample, such as the fraction of failing die per wafer or rejected parts per shift. Limits are p-bar plus and minus 3 times the square root of p-bar times (1 minus p-bar) divided by n, so they step in and out when sample sizes vary. The np chart plots the count instead of the fraction when n is constant; c and u charts handle counts of defects rather than defective units.
Final visual reject rate is up on the p chart, but sample size dropped to 50 on nights, so check the limits widened too.
Also heard as
- fraction defective chart
Related terms
Control chart
Time-ordered plot of a process statistic with a centerline and limits that flag when the process has changed.
UCL
Upper control limit: the top boundary on a control chart, paired with the lower control limit (LCL) below the centerline.
Acceptance sampling
Accepting or rejecting a whole lot based on inspecting a random sample from it against a predefined plan.
Yield
Fraction of units that come out good at a step or across a whole process, the headline number for any production line.