x-BAR, p or U chart? Plastics compounding line

D

davis007

#1
I have a set of data taken for the year. The data is from a plastics compounding line and shows the lbs of prime material produced, lbs of fines (one type of scrap) produced, and the lbs of startup and shutdown (second type of scrap) produced by week. This data is not taken from a sample but is the actual measurement for the entire week. We produce several different grades of material and have this data for each grade. But the total produced can vary widely from week to week.

To see if our process is in control with regard to how much scrap we are producing what type of chart should I be using.

An individuals chart were the data is % of fines and % of Misc for each week?

A P chart were the data is total lbs of fines produced, total lbs of misc produced and total lbs produced (fines+misc+prime)?

I have plotted the data both ways and arrive at completely different conclusions. The Individuals chart shows the process to be in control for the year, 1 assignable out of control point. While the P chart shows the process to be completley out of control.

Thanks for any help.
 
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#2
Re: x-BAR, p or U chart?

A P chart were the data is total lbs of fines produced, total lbs of misc produced and total lbs produced (fines+misc+prime)?

I have plotted the data both ways and arrive at completely different conclusions. The Individuals chart shows the process to be in control for the year, 1 assignable out of control point. While the P chart shows the process to be completley out of control.

Thanks for any help.
The difficulty you are having is that the p-chart is based upon a series of go no-go trials. I flip a coin and it is either heads or tails. Unfortunately, in your data, a pound of material is not a single coin flip. There may be multiple coin flips within the pound of material. Or perhaps the pound of material is part of a batch of 100 pounds of material that all went to one status due to one event. In a situation like this, the p-chart is not appropriate, the predicted variation will not match your process, as you discovered. An x type of chart is more appropriate here because I do not have a theoretical variation for the data.
 
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