A new process is started, and the sum of the sample standard deviations for 25 subgroups of size 4 is 750. If the specifications are 700 +- 80, what is the process capability index?

Question

What I know:
D2=2.059
USL = 780; LSL = 620; Tolerance = 160

Answer 1

Cp=USL-LSL/6 sigma note
= 160/6sigma note
sigma note=s(bar)/c4 ; c from the table for sybgroub size 4
c4= 0.9213
s(bar)=750/25=30
sigmanote=30/0.9213=32.56
6 simgma note=6* 32.56=195.37
so
cp=160/195.37=0.818
cp<1 , the process in state I