Comparing PPM Year over Year - Determining the PPM decrease from 2009 to 2010

C

CMfgT

#1
Can some one check my logic?

I am trying to determine the PPM decrease from 2009 to 2010.
The department PPM in 2009 was 11,754
The department PPM in 2010 was 8,638

So wouldn't this be a 27% decrease in PPM

((2010PPM-2009PPM) / 2009PPM)*100

I asummed this was like sale increases and decreases?

thanks,
 
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B

Bill Pflanz

#2
Re: Comparing PPM year over year

Or it could just be normally variation. If you have the monthly data, control chart that data and it will be more meaningful.

Bill Pflanz
 

Jim Wynne

Staff member
Admin
#3
Re: Comparing PPM year over year

Can some one check my logic?

I am trying to determine the PPM decrease from 2009 to 2010.
The department PPM in 2009 was 11,754
The department PPM in 2010 was 8,638

So wouldn't this be a 27% decrease in PPM

((2010PPM-2009PPM) / 2009PPM)*100

I asummed this was like sale increases and decreases?

thanks,
What's wrong with just comparing the number of defectives for each year?
 
C

CMfgT

#4
Re: Comparing PPM year over year

What's wrong with just comparing the number of defectives for each year?

Can't compare defectives because they made a larger amount of product, then I wouldn't be comparing apples to apples.

As far as control charting. I am not trying to make this a hard to understand, I just want to show the operators the PPM dropped by X percentage and tell them thank you for all of the hard work. We have been decreasing our PPM and we continue to grow.

Thanks,
 
B

Bill Pflanz

#5
Re: Comparing PPM year over year

Based on your comments the ppm decrease calculation is fine to use. Just be careful not to oversell it if it is just normal variation.

Bill Pflanz
 
T

tomvehoski

#6
Re: Comparing PPM year over year

I've never like PPM measures, as all of the following could be true.

2009 PPM resulted in a paper cut to the user in every instance.
2010 PPM resulted in death to the user in every instance.
We reduced PPM by 27% from 2009 to 2010.

I don't think you can claim improvement in the above (hopefully exaggerated) example. Of course, you can do the same with any statistics.
 

Jim Wynne

Staff member
Admin
#7
Re: Comparing PPM year over year

Can't compare defectives because they made a larger amount of product, then I wouldn't be comparing apples to apples.
I should have asked the question more clearly. Why can't you just use simple ratios (x% in 2009, y% in 2010) and leave the problematic PPM thing out of it?
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#8
Jim - I'm not sure what is problematic about this particular situation. The percent change doesn't change if you use ppm or %.

(assuming that the ppm is not some theoretical statistical estimate)

after all % is the proportion multiplied by 100 and ppm is the same proportion X10^6...both measures make the printed number look bigger than the proportion.

(I have the same caveats concerning random noise and more importantly the issue of whether or not the 'operators' had any influence on the results. But those aren't effected by the concern you raise)
 

Jim Wynne

Staff member
Admin
#9
Jim - I'm not sure what is problematic about this particular situation. The percent change doesn't change if you use ppm or %.
I understand. A ratio is a ratio. My misgivings are with the idea of PPM, which makes no sense if you're not actually making millions of things. Why not PPT--parts per trillion?
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#10
My misgivings are with the idea of PPM, which makes no sense if you're not actually making millions of things. Why not PPT--parts per trillion?
Well, if he is making millions of parts then there isn't an issue?
one of my product lines measures things in ppb. because we really are looking at billions. so I guess if I were measureing or making trillions I'd use ppt
 
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