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C5. Tips in Selecting a Control Chart
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hi welcome back this is the last topic
under control phase selecting a control
chart it may seem very easy to select a
control chart but we have clients that
find it very difficult and even
frustrating to finally pick the right
control chart for their specific process
I even had my first consulting client
who have used a wrong control chart for
the past five years having so many data
points outside the control limits even
to the frustration that they cannot find
the reasons of the special process and
with my help and was able to correct the
way they use the control chart now this
is the reason why we want to teach you
how to select the right control chart in
selecting the control chart it's very
important for us to know it's very
important for us to know our data types
we have again continuous and attribute
now continues if you can see remember
talks about anything that can be
measured while attribute talks about
anything that can be counted
this is based on categories now let's go
to continuous data if you want to chart
your data but you have limited number of
samples let's say one sample per
subgroup you can now use what we call
individual moving range chart this is
use when your process gives an output
that is hard to measure or is expensive
to measure usually we have data
collected in subgroups this is divided
into either eight samples or less in a
group when you can use what we call the
x-bar our chart or the mean and range
chart if you have more than samples than
eight now we go to what we call X bar s
chart or mean
and standard deviation chart let's see
if you are in a buckling company and you
want to test whether your process yields
a statistically process controlled
output and you take one bottle measure
its diameter and records it every day
then probably you have to use individual
moving range chart because you have one
sample per D and your data is continuous
that is diameter now if in the same
process your team decided that you have
to collect five samples measure the
diameter plot in a control chart then
probably you're using an x-bar and our
chart because you have five samples of
group size which is less than eight now
if you decided to collect 15 samples per
subgroup size measure the diameter and
plot it in a control chart definitely
you will be using X bar and s chart
these are the three charts that we use
under continuous data take note that
continuous data is also called variable
data now let's go to the other side we
have here attribute data anything that
can be counted based on categories if
you are counting defective items then
you have an option to use what we call P
chart or proportion chart and if you are
counting defects per unit then probably
you have to use what equal you chart now
before we go to the charts let's first
discuss what is the difference between a
defective item and defects per unit when
can we say that an item is defective and
when can we say that an item or service
has a defect let's put it simply if the
primary function of a product or service
cannot be performed then it is defective when
when
this condition does not hold true then
this Prada
or service has a defect look at your
cell phones if your cell phone has a
dent you cannot say that this defective
you can say that there is a defect in
the ceiling you can seduce your phone
for calling or texting someone but if
your phone has a problem with the
motherboard and you cannot turn it on it
cannot function as a calling and texting mechanism
mechanism
therefore it is now defective so that's
the difference between defective item
and defects per unit now let's put this
control chart selection in a simpler
form I have here prepared two tables if
your data is continuous that is anything
that can be measured you will check your
sample size on the Left column match it
with this categories and you can now
identify whether it will fall in an
individual X moving range chart or in an
X bar and our chart or in an X bar and s
sharp that's simple if your data now is
an attribute data anything that will be
counted from categories you have to ask
yourself two questions first what are
you counting if you are counting
defective items coming from the samples
the next question would be is your
sample size fix meaning for every time
that you take samples the sample size
does not change let's say every day you
have to take five samples then it's a
fixed sample size if that's the case you
will have to use NP choice on the other
hand if you are counting defective items
from samples that you are doing coming
from the production or service facility
and the sample size is variable or a chain
chain
just from time to time let's say day to
day you cannot use any chart anymore but
rather you have to use what we call a
chart these are the two charts that you
use when you're counting the effective items
items
now let's say you're counting defects
per unit the next question will be is
your sample size fix if yes then you
have to use what we call a seat sir and
if your sample size is variable for
varying you have to use what we call
future this is a simplified version of
how you can use control chart in order
for you to have better results in
managing and monitoring your process
output in terms of the variation
remember that in your controls control
plan you can use control chart for you
to be able to monitor its performance as
against the required or necessary amount
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