Capability Studies - Combining parts from two lots

[Dawn]

If we have a capability problem and we need to do a capability study, can we get an accurate picture of taking 6 pieces from one lot (that's all we have) and 6 pieces of another lot and combining them together and doing a study on those 12 pieces or would it be inaccurate because of two different lots?

[Kevin Mader]

Dawn,

Combining the two lots would create a bimodal distribution which introduces bias to your calculations. I believe that the answer is no. Don Winton (and I believe Howard Atkins?) dabble in the Advanced Statistics world and are probably a better source for a solution to your problem than me. I will offer my thoughts if no one can come up with anything.

Answers anyone?

[Dawn]

Kevin,
Even if they were ran at the same time?

[Kevin Mader]

Dawn,

If they were run at the same time, material source was identical (i.e. same coil of steel used on the same machine with the same initial set-up) then you should be alright. Essentially all you have done was to split a single population into two similar lot populations. They should have the same properties. If you can be sure by the records that this is your case, then combining the two sample lots is probably fine (this is assuming that the distributions are normal and that they have similar standard deviations).

I was really hoping that one of the big guns would bail me out and give you something a little more solid for you. But I hope this helps you some what. Happy Holidays!!

[Dawn]

Kevin,
It appears to me you are one of the big leagues.
Thanks for the help - I will use your information as a yes when I go back to work on Monday.
Merry Christmas and Happy New Year!

[Don Winton]

Dawn,

You should not have a problem using the two different lots as long as they are representative of the process.

The better answer, of course, is to select lots that are current and running and in a state of statistical control. "THERE IS NO CAPABILITY WITHOUT CONTROL." Ott and Schilling, Process Quality Control, 1990. I assume you already knew that.

Kevin’s point about the bi-modal effects is correct as far as ‘lots’ go and care should be taken. Your observations concerning the ‘lots’ would seen to indicate they came from the same ‘run’ in which case you should be OK and Kevin’s responses are accurate.

“Essentially all you have done was to split a single population into two similar lot populations.” Kevin, more correctly stated, “Essentially all you have done was to split a single population into two similar lot samples.” Just a small point. :>)

I apologize for not joining this thread sooner. The ice storm that blew through here wrecked havoc on power. I have just now been restored and do not know how much longer it will last.

Regards,
Don

[Kevin Mader]

Don,

Thanks for the bail out. It is appreciated from this end. Happy New Year by the way!

Regards,

Kevin

Don correctly pointed out that a capability study is valid when a process is in a state of statistical control (stabililty).

If Dawn were to use the 12 pieces and plot them on a an SPC chart, what chart should she use? An average and range chart could support n=2 and only 6 points to plot - not enough to show out of control conditions like 8 on one side, 8 trending and so on. An moving average and moving range may be suitable (described in Besterfield's Quality Control Book) to detect instability.

Also, could the 12 measurements be placed in a histogram and an examination for normality be suitable?

Just some more thought on the subject.

Kelly Speiser