<s>
In	O
statistical	O
quality	O
control	O
,	O
the	O
and	O
s	O
chart	O
is	O
a	O
type	O
of	O
control	B-General_Concept
chart	I-General_Concept
used	O
to	O
monitor	O
variables	O
data	O
when	O
samples	O
are	O
collected	O
at	O
regular	O
intervals	O
from	O
a	O
business	O
or	O
industrial	O
process	O
.	O
</s>
<s>
The	O
"	O
chart	O
"	O
actually	O
consists	O
of	O
a	O
pair	O
of	O
charts	O
:	O
One	O
to	O
monitor	O
the	O
process	O
standard	O
deviation	O
and	O
another	O
to	O
monitor	O
the	O
process	O
mean	O
,	O
as	O
is	O
done	O
with	O
the	O
and	O
R	O
and	O
individuals	B-Application
control	I-Application
charts	I-Application
.	O
</s>
<s>
As	O
with	O
the	O
and	O
R	O
and	O
individuals	B-Application
control	I-Application
charts	I-Application
,	O
the	O
chart	O
is	O
only	O
valid	O
if	O
the	O
within-sample	O
variability	O
is	O
constant	O
.	O
</s>
<s>
Therefore	O
,	O
several	O
authors	O
recommend	O
using	O
a	O
single	O
chart	O
that	O
can	O
simultaneously	O
monitor	O
and	O
S	O
.	O
McCracken	O
,	O
Chackrabori	O
and	O
Mukherjee	O
developed	O
one	O
of	O
the	O
most	O
modern	O
and	O
efficient	O
approach	O
for	O
jointly	O
monitoring	O
the	O
Gaussian	B-General_Concept
process	I-General_Concept
parameters	O
,	O
using	O
a	O
set	O
of	O
reference	O
sample	O
in	O
absence	O
of	O
any	O
knowledge	O
of	O
true	O
process	O
parameters	O
.	O
</s>
