Douglass Index Details

The Douglass Index serves to ensure that you fully analyze your operational data. If you encounter variations in plots of annual study cycles, existing machine qualifiers (required before a machine is used), supplier product monitoring (necessary when there are multiple suppliers on your chain), and machine re-certifications (done to verify that your machine's condition falls within specific criteria), calculating the Douglass Index will save you time.

In statistical process control, technicians and auditors administer and regulate detailed work plans to gather data for quality engineers. These engineers then crunch the data into capability studies (calculations done to evaluate if a process meets the requirements). Then they determine the repeatability and reproducibility (R&R) rates for the gauges (the instruments that display the amount or the level of something) used in the capability studies. Repeatability weighs the disparities in the results received by a person using the same instrument. Reproducibility measures if someone can replicate a study.

The lower the repeatability and reproducibility rate (R&R rate), the better is the measuring system. R&R rates that fall under 10% are spanking, and those that fall between 10% and 30% are acceptable. In cases when it's difficult for the rate to fall below 10%, the Douglass Index formula can relate a low R&R rate (falling between 10% and 30%) to a marginal capability index (Cp). They do this by increasing the capability index to an improved capability index.

Douglass Index Example

Begin by looking at the study results and listing each R&R rate on a table. The Douglass Index equation subtracts the square root of the sum of the squares of the instrument blunder from the actual part deviation. It also deletes whatever makes the variation of the part larger so researchers can approve the data.

If you measure an item using a sensitive gauge that cumulates a high bias, you will get a marginal-to-poor R&R rate. The technical team will then analyze the results and identify incapable processes or capability failures.

For example, a capability study with the desired target of 1.71 or greater has returned a failing Cp of 1.63. The technicians would then compare this to the Douglass Index conversion table to learn if the R&R rate is 25% or less. If so, the index would convert the failing Cp of 1.63 to an improved 1.71.

Significance of the Douglass Index

Technicians using the Douglass Index can save money each time they salvage a Cp through the conversion tables. This way, they manage to discard the need for re-study and allow consultants and auditors to avoid the time-consuming rework and begin collecting data on their next assignment.

Technicians warn that we should avoid applying this formula in new program launches and new machine trial runs. Keep in mind that the Douglass Index works best when determining annual study cycles, evaluating existing machine qualifiers or supplier product monitoring, as well as when looking to obtain machine re-certifications.