<s>
In	O
statistical	O
process	O
control	O
(	O
SPC	O
)	O
,	O
the	O
and	O
R	O
chart	O
is	O
a	O
type	O
of	O
scheme	O
,	O
popularly	O
known	O
as	O
control	B-General_Concept
chart	I-General_Concept
,	O
used	O
to	O
monitor	O
the	O
mean	O
and	O
range	O
of	O
a	O
normally	O
distributed	O
variables	O
simultaneously	O
,	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
(	O
as	O
approximated	O
by	O
the	O
sample	O
moving	B-General_Concept
range	I-General_Concept
)	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
s	O
and	O
individuals	B-Application
control	I-Application
charts	I-Application
.	O
</s>
<s>
As	O
with	O
the	O
and	O
s	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>
