Hi, I'm Ray Sheen.
Let's continue our discussion of Gage R&R by looking at the step by
step approach for planning and conducting a Gage R&R study.
I'll start with an overview of the three phases of a Gage R&R study.
The first phase is the planning phase.
The keys to this are to focus the study, create the plan and
then make your preparations.
While the third of these, sounds the simplest,
I actually find it the most difficult because of the work to create items for
measurement and lining up the resources.
Once you are ready, you can conduct the plan,
this involves measuring the items and recording the data.
Based upon the difficulty of the measurement system and
the ability to gain access to resources, this may take an hour or several weeks.
The third phase is the analysis, this is very different depending upon whether
the data was variable data or attribute data.
The calculations are complex enough, that we'll devote an entire
lessons to each of these but for now, let's dig a bit deeper into each phase.
Let's start with the creation of the Gage R&R study plan.
The first step is rather obvious.
Decide what measurement you're interested in.
We need to understand if it is a variable or attribute measurement.
While this step seems obvious,
it's not a trivial question on a Lean Six Sigma project.
It's easy to do a study on what is convenient to measure but
you need to ask what is important to measure and how will we measure that.
The next step is an obvious follow from the previous step.
Now that we know what we're measuring, the question becomes how it is measured.
What is the current measurement system in process?
Where is the measurement done?
What equipment or procedures are used?
Who does the measurement?
If there's no existing measurement process, you'll need to create one and
then your Gage R&R study becomes part of the validation process for
that measurement system.
These first two steps will focus our efforts.
Step 3 is to design the study.
That means deciding how many appraisers, items and trials.
There are some differences in the approach, depending upon,
whether it's variable measurements or an attribute measurement.
With variable studies, use 5 to 10 items.
With attribute studies, use 20 to 30 items.
The easiest way to see the study design is to see the data collection form.
That's where all of the design parameters are captured.
I’ll show these forms in the next few slides. Which takes us to step four.
With the study designed and
the forms ready, we need to communicate the plan to everyone involved.
Normally this is quite easy but depending upon what's involved,
you may need to do things like make special sample items for
inspection that are known to be out of specification.
So there may be some cost and time needed to create those items.
The coordination step is often combined with the previous step.
It is a coordination of the resources such as facilities equipment, parts for
inspection and the people needed to run the study.
When communicating the plan you may wanna start this coordination.
For instance, you may need to get access to the measurement equipment that
is being used as part of day to day business operations.
You'll need to coordinate them with the area supervisor.
Also, the operators who will be the appraisers need to be identified, so
that they're relieved from normal duties and available when you run the study.
Depending upon the complexity of the measurement system,
you may need support from other organizations.
This is especially true if you have to conduct the study outside of normal
business operations such as on a weekend.
Finally, you need to train the operators in the purpose and process of the study.
The individuals will need to understand that the study is not a performance
appraisal on them, it is an evaluation of the system.
Sometimes they get nervous about the study and
that creates special cause variation problems in the results.
Of course, if this is a brand new measurement system or
process, the appraisers will need to be trained on how to use the equipment.
Lets now look at the data collection form for variable data, Gage R&R study.
If using Minitab, a statistical software application commonly used
with Lean Six Sigma projects, this is easy.
You just enter your study parameters into the mini tab assistant setup form and
it generates a study plan complete with the data collection form.
I'll show you how to do this in a later lesson where I also show you how to use
Minitab to analyze the data.
The measurement results from each measurement are then entered right
into the Minitab form and you're ready for the analysis.
But if you don't have Minitab,
don't worry, you can do everything in the spreadsheet such as Excel.
You will need to create your own data collection form and
you'll need to put in the equations for your analysis.
Again, I'll show you how to do those on the variable data Gage R&R lessons.
As you can in this form, for this study we'll have three appraisers
who each will measure 10 items and do these three times.
One of the things that I have also put on this form is that we'll need to calculate
the averages or means for each appraiser and each part.
We'll use those in the calculations.
One caution when planning the study.
You need to do the measurements in a random order but
you need to record the data for the correct item.
So to prevent the appraiser knowing which item they're measuring and
their previous measurement value,
I always have someone else record the data who knows which item is which.
Well, that was for variable data studies.
The attribute data Gage R&R data collection looks a little bit different.
The Minitab approach is the same. Enter your study parameters and
Minitab will create the plan and the data collection form.
And I'll show you how to do that in the attribute data Gage R&R lesson.
And like with the variable data, we can do this analysis in a spreadsheet,
we just need to manually create the form and enter the equations.
As you can see, the form is different for attribute data.
We don't have numerical measurements.
Instead, we have pass-fail or good-bad ratings for each item.
Once again, I have three appraisers.
And this time, just two trials are repetitions.
But with the attribute data, you need more data points.
So my spreadsheet goes all the way down to data item number 30.
There are also columns for matches and match to the standard.
I will discuss those in the lesson on the Attribute Data Gage R&R calculation.
One of the things about Attribute Data Gage R&R analysis,
is that it can include an assessment of the effectiveness of the measure,
in addition to repeatability an reproducibility.
To do that, we need to know the true value of each item.
So, we have a column for
the actual condition of the item labeled standard that's either good or bad.
Once again, the items need to be randomized in the order of inspection.
But the correct value needs to be entered for each item.
So I have someone else recording the data.
The methodology for planning and executing a Gage R&R study is not difficult.
The key will be the creation of the data collection form which
captures the planning and sets us up for the data analysis.