What if DOE (Design of Experiments) is not possible?

Jim Wynne

Staff member
Admin
#11
Dear all,

I'm approaching the world of Six Sigma and during my first readings I came up with a doubt.


I understood that I can use DOE in order to find the y=f(x) function for multiple x. Different techniques are available depending on the number of x and other parameters.

My question is... what happens if I can't perform DOE. How can I determine the most influencing Xs and consequently the f(Xs) function in case of multiple inputs?

I hope I'm not too confused...
Thank you.

Giuseppe:bigwave::thanx:
Is this is hypothetical situation/question or an actual problem you're facing?
 
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BradM

Staff member
Admin
#12
Brad, thank you for the excellent answer.
So I understand that if I can't carry out DOE's I have to apply a regression analysis in order to obtain the f(X) and to understand how the measured Xs influence the Y.
Any suggestion about docs or web material to read on this subject?
I first strongly suggest you read my colleagues posts. We are certainly not in your situation, and that is why it's always good to review all the options.

Among them, allow me to suggest second reading Steve's post. This may be an ideal situation for Control Charts. If you are interested in that, I have a link to his profile. Follow his posts, and feel free to PM him. He is a really nice guy, and will be more than happy to help you.

However, just so you have multiple options:

In simple terms, you are wanting to find relevant correlations for your process.

Here is a Wiki on Regression:
http://en.wikipedia.org/wiki/Regression_analysis

With your inputs, is there a measurable trend? When your input changes, what happens to the output? Regression (and even, to an extent, correlation) is for prediction. You are trying to say something like "Ok, if I were to have .......... as inputs, what (with some reasonable level of confidence) would my output be?

Notice, that is a totally different function than say Control Charts, and Steve's suggestion. This is why Jim queried what is your application, and exactly what you are trying to accomplish. We're here to help; so we don't want to send you on your way with the wrong tools and unapplicable information.
 
P

pagnonig

#13
I first strongly suggest you read my colleagues posts. We are certainly not in your situation, and that is why it's always good to review all the options.

Among them, allow me to suggest second reading Steve's post. This may be an ideal situation for Control Charts. If you are interested in that, I have a link to his profile. Follow his posts, and feel free to PM him. He is a really nice guy, and will be more than happy to help you.

However, just so you have multiple options:

In simple terms, you are wanting to find relevant correlations for your process.

Here is a Wiki on Regression:


With your inputs, is there a measurable trend? When your input changes, what happens to the output? Regression (and even, to an extent, correlation) is for prediction. You are trying to say something like "Ok, if I were to have .......... as inputs, what (with some reasonable level of confidence) would my output be?

Notice, that is a totally different function than say Control Charts, and Steve's suggestion. This is why Jim queried what is your application, and exactly what you are trying to accomplish. We're here to help; so we don't want to send you on your way with the wrong tools and unapplicable information.
Dear all,

first of all I would like to thank you for your precious support.

In any case, this is not a situation I'm currently facing. It's an hypotetical situation.
I understood (I hope!) after some readings that DOE is a tool to understand which inputs (X) in a selected group are more significant than others.
DOE analysis should also detect the relationship f(x) between the significant inputs and the output in order to be able to determine the right x values to obtain the desired Y (thus improving the process).

If the above is correct, imagine a situation where I can't carry out an experiment for any reason.
Then I ask myself, could I use the Xs and Y readings of the process during his normal runs in order to carry out a similar analysis (I mean giving me more or less the same indications) as DOE?
If yes (regression?) do I have to use continuous data from the process or I can sample it?

I hope I have better defined the situation.
Forgive me if I'm not clear enough, please ask for any clarification if needed.
Thanks a lot!
Giuseppe
 

Jim Wynne

Staff member
Admin
#14
In any case, this is not a situation I'm currently facing. It's an hypotetical situation.
It would have been better to state this in the original post. We often have people who are in a production situation and need quick answers, which we are usually able to supply. :D

I understood (I hope!) after some readings that DOE is a tool to understand which inputs (X) in a selected group are more significant than others.
DOE analysis should also detect the relationship f(x) between the significant inputs and the output in order to be able to determine the right x values to obtain the desired Y (thus improving the process).
In order for any designed experiment to be efficacious, you need to be able to control and isolate the variables. In actual production situations, if it's not possible to stop the show and do your experiment, it's often a case of trial and error. Furthermore, experience often obviates the need for formal experimentation--we can predict based on empirical knowledge, and then "tweak" as necessary. There's almost never a need for formal experimentation on a running process that's stable and producing conforming output.

If the above is correct, imagine a situation where I can't carry out an experiment for any reason.
Again, if you can't control and isolate the variables, you can't do a formal experiment. You can control and isolate the variables by doing adjustments while the process is running, but you have to be prepared for nonconforming output. It seems that many SS tyros have been taught that in order to improve processes you must do formal experimentation and you must do MSA and you must engage in any number of other potentially wasteful activities in order to achieve improvement. It ain't necessarily so. It's not enough to have a big tool box; you have to know when it's appropriate to use each tool.
 

BradM

Staff member
Admin
#15
...If the above is correct, imagine a situation where I can't carry out an experiment for any reason.
...Giuseppe
Guiseppe, allow me to expand a bit on Jim's previous post, so I don't add too much confusion to the picture. Imagine you have two machines kicking out widgets (Y):

Machine 1 you can manipulate the different factors involved (X). This is a basic, classic experiment.

Machine 2 you cannot manipulate any of the factors (X). However, you can measure the X's and Y's. You can then make inferences about the Y's based on the X's. You gain results, but it is not an experiment per se. An experiment again in manipulating one variable to observe the effect on another variable.

Again, Steve's and Jim's point about stability and control in your process will probably be more relevant for you, IMHO. Unless you have a situation where you need to predict, the simplest approach would be establishing stability and control in your process, and monitoring it.
 
P

pagnonig

#16
Ok, I think I catch the point.
Jim, I apologize for the misunderstanding about the real/hypotetical situation.

This was my first post, it's really a GREAT forum!
Thank you for your support. Giuseppe

PS: I will post a request for suggestions about Six Sigma exercises material... of course you are invited:)
 

Jim Wynne

Staff member
Admin
#17
Ok, I think I catch the point.
Jim, I apologize for the misunderstanding about the real/hypotetical situation.

This was my first post, it's really a GREAT forum!
Thank you for your support. Giuseppe

PS: I will post a request for suggestions about Six Sigma exercises material... of course you are invited:)
No need for apologies, and welcome to the Cove.:D
 
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