Calculations to create PPM charts
PPM is short for Parts Per Million. A PPM Chart
reports data in units of parts per million.
The most direct way to get a chart like this
is to convert your data to the proper units,
and then display the chart.
For example, take a look at the data we include
with Statit in the workspace file, pvarn.wrk.
Read this data into Statit using the Open Workspace
dialog or the GETFILE
command and you will find three variables. The
Date variable identifies the day on the which
the data was collected. On the given day, a
number of pieces were tested and this number
was put in the Inspected variable. Of those
pieces, a certain number failed the test and
this number was put in the Rejected variable.
The data looks like this:
A p Chart is appropriate for looking at this
data. The data variable is the number rejected
and the subgroup size is the number inspected.
You can have the p Chart display either the
fraction or the percent rejected. Try these
Rejected by Inspected /axis=Date
gpchart Rejected by Inspected /axis=Date /percent
As this shows, the data we have gives us the
fraction of the total inspected that were rejected.
To display this as the number rejected in units
of PPM, we first convert the data to these units.
This is done by the following expression:
PPM = (Rejected / Inspected) * 1e6
This creates the variable PPM, which contains
the fraction of 1,000,000 pieces (PPM) which
would have been rejected if there were 1,000,000
pieces inspected every day. (1e6 is a short
way of expressing 1,000,000.) Since we now have
this calculated value, we use an X chart (also
called I or Individual or Run chart) to display
ppm by 2 /axis=Date /ymin=0
Note that we specify a minimum Y value, since
this attribute data will not go below 0. Notice
that the control line, which is calculated,
is now straight because the subgroup size is
constant. We are not concerned with specifying
the 2 for the subgroup size on this command
because the subgroup size is already included
in the calculation of the PPM values and is
used here for the control limit calculation.