Answer:
Step-by-step explanation:
Given that,
μ = 25.1
σ = 0.26
a) since standard deviation is ideal measure of dispersion , a combination of control chart for mean x and standard deviation known as

Chart is more appropriate than
and R - chart for controlling process average and variability
so we use
charts
b)
n = 11
we have use 2 σ confidence
so, control unit for
chart are
upper control limit = 
lower control limit = 
control limit = μ
μ = 25.1
upper control limit =

lower control limit =

Upper control limit and lower control limit are in between the specification limits , that is in between 24.9 and 25.6
so, process is in control
c) if we use 3 sigma limit with n = 11
then
upper control limit = 

lower control limit = 

control limit is 25.1
Then, process is in control since upper control limit and lower control limit lies between specification limit
So, process is in control