Having repeatability and reproducibility rates that are 10% or lower is ideal for applying gauges in capability data retrieval. If your R & R rates are above this level, there is a powerful tool for managing these prohibitively large rates.
Working with the Douglass Index allows auditors to crunch data that may fall outside the desirable repeatability and reproducibility rate.
Overview: What is the Douglass Index?
The Douglass Index is a method that can determine if data has been exhausted in an annual study cycle. It also serves as an indicator as to whether the data needs to be redrawn.
3 benefits of the Douglass Index
There are some major benefits of the Douglass Index that are worth exploring:
1. Saves time
Calculating the Douglass Index will save time if you encounter variations in machine re-certifications, machine qualifiers, or annual study cycles.
2. Ensures full analysis
This index ensures the full analysis of operational data.
3. Improves the capability index
In situations where it is difficult to drop the repeatability and reproducibility rate below 10%, the Douglass Index can relate this low rate to a marginal Cp.
Why is the Douglass Index important to understand?
The Douglass Index is important to understand for the following reasons:
It can save money
Understanding The Douglass Index can save time for technicians whenever they can use it to salvage a Cp.
It can eliminate re-study
Having a working knowledge of the Douglass Index can eliminate the need for a re-study of a Cp.
Having an understanding of the Douglass Index can save you the time needed for rework so that you can move on to collecting the data for the next project.
An industry example of the Douglass Index
An overworked auditor is busy crunching the latest set of data for their organization’s quality engineer. It comes to the attention of the auditor that the data set is falling upwards of 40% in its repeatability and reproducibility rate. This is nowhere near the rate needed to be able to present the data for the engineer to use the data for a capability study. Instead of panicking about having to rework the data, the auditor applies the Douglass Index, which brings the rate into a range that is acceptable.
3 best practices when thinking about the Douglass Index
When working with the Douglass Index, here are some key practices to keep in mind:
1. Where to avoid the Douglass Index
The Douglass Index should be avoided in the trial runs of new machines and the launches of new programs.
2. Repeatability and reproducibility rate
Contrary to the intuitive view that a higher percentage in this instance would be better, the lower your repeatability and reproducibility rate is, the more ideal the measuring system.
3. Use the Douglass Index to segregate equipment variation
You can use the Douglass Index not only to improve a capability Cp but also to segregate equipment variation.
Frequently Asked Questions (FAQ) about the Douglass Index
In what R & R rate range is the Douglass Index not necessary?
If an R & R rate is under 10%, that is considered ideal. If the rate is between 10% and 30%, it is often considered acceptable.
How does the Douglass Index equation work?
It subtracts the square root of the square sums of gauge inaccuracy from the true part variation. This removes what creates a falsely larger part variation.
What assumption does this formula make?
It assumes that Cp and Cpk are equal, so there is not an issue with targeting.
Working with the Douglass Index
The Douglass Index can ease the stresses that go with finding your data falls outside the suitable repeatability and reproducibility rate. It can also save a significant amount of time and money by cutting out the need to rework data. For these reasons, this index should be in the arsenal of all auditors and technicians who are working with statistical process control.