In measurement system analysis (MSA), we would like to
accomplish accuracy and precision of the ,measurement data.
Accuracy can be achieved through
performing calibration on the measuring device for variable GR&R and clear
operational definition for attribute GR&R.
Precision can only be achieved through GR&R studies followed by
improvement of the measuring process if the GR&R does meet the required
standard. Calibration and GR&R
studies are completely different. Some
manufacturers tend to mix up both
calibration and GR&R studies. There
was once an established supplier show me a calibration record when I ask for GR&R
studies report????!!!!
GR&R studies need to have a lot of data
collection, very often most suppliers or
manufacturers choose to ignore conducting GR&R studies unless it is
requested by customer. After working
with over 100 electronic parts suppliers,
most of them will jump straight
into actual data collection without bothering about accuracy and
precision of their measurement system. They did not
realize that without GR&R studies there is no way to know if the data they
are collecting is reflecting the actual process variation. GR&R
study is a must for new measuring device or after the device had undergo major
repair or move to another location. In
some industries GR&R study is required in new product qualification where
GR&R report should be included in First Article Inspection (FAI) Report.
Although measurement process is one of 6 process input
(5Ms and 1E), however it is NOT the
actual variation from process input. The variation in measure comes from measurement process and
not the product itself. We would like to
reduce variation contributed from measurement process to be a small as possible
so that our observed data will show more actual process variation. Ideally in variable GR&R we only want
< 20% of total process variation come from measurement input per item
5. Therefore if GR&R value is above
the acceptable level then we must improve the measurement process.
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The correct method to conduct GR&R is also very
important right from operational definition to selecting sample to interpreting the results. For those who
had already attended my training class before would know the proper steps to
obtain accurate GR&R value.
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Without GR&R study we will never know if the data
collected show is the actual process variation or measurement process
variation. There are cases where
measurement process variation contributed more than 50% of the total process
variation. In this case we will only need to improve the measurement process
rather than the actual process improvement!
If the measurement data is not accurate and/or precise, it will lead us to
the wrong action and conclusion. We
could do a lot process improvement to man, machine, method or material and yet
the data does not reflect the improvement we made due poor measurement process.
In summary we have to conduct GR&R studies with the
purpose
- Quantify the precision of the Measurement System
- Check for measurement system integrity
- Ensure our data from measurement data does NOT have error which cause us to make wrong decision
Click to get the link to my course
Do refer to my course for more information on setting up effective measurement system
Therefore I hope that after reading this article, my reader will make some effort to conduct
GR&R studies before any process improvement initiatives to ensure that
process variation is NOT contributed from measurement process.
