Calculating PPM - Multiple product lines and multiple type of parts

[56flh]

When your company produces multiple product lines that run from intricate custom machined assemblies to screw machined parts off of a multi-spindled automatic, how do you calculate PPM?

I have received questionaires from customers with a space to enter one number and then other spaces for improvement anticipated for the next few years. Some of these customers we sell the above mentioned diverse amount of products. This one PPM number I have to enter would be totally different based on those large, intricate items we may sell 1000 pieces of vs screw machined products that we sell hundreds of thousands of pieces.

At this point I am baffled by the validity of six sigma calculations.

 

[bobdoering]

Most likely you have customers rubber stamping rudimentary quality notions trying to meet some either self- or customer-imposed criteria.
It is equivalent to asking McDonald's "How much food do you sell?" What? Does that include drinks? In units? In lbs? It's "over-the-bow" statistics (using the term 'statistics' loosely), right up there with Cpk.

If they do not specify the ppm calculation, assume ppm rejected of shipped product. Then, identify your goals for improving upon that (if not already zero).

At this point I am baffled by the validity of six sigma calculations.

You are not alone...

 

[Statistical Steven]

When your company produces multiple product lines that run from intricate custom machined assemblies to screw machined parts off of a multi-spindled automatic, how do you calculate PPM?

I have received questionaires from customers with a space to enter one number and then other spaces for improvement anticipated for the next few years. Some of these customers we sell the above mentioned diverse amount of products. This one PPM number I have to enter would be totally different based on those large, intricate items we may sell 1000 pieces of vs screw machined products that we sell hundreds of thousands of pieces.

At this point I am baffled by the validity of six sigma calculations.

This is using Six Sigma, but just math.

Part Defects Volume % Defective PPM
1 3 5000 0.06% 600
2 7 5000 0.14% 1400
3 40 10000 0.40% 4000
4 2000 160000 1.25% 12500


Combined 2050 180000 1.14% 11389
Average 4625

So combining the data leads to an overall PPM rate of 11389. If you want to use the average PPM across all products it would be 4625. I believe the combined approach is best.