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Oct 12, 2011 2:54 AM
(1276 views)

Hi All,

Im fairly new to JMP and im in the process of setting up a DOE to look at the effects of several ingriedients within a mixture on a specific response. Im only interested in the main effects and the two way interactions. Do I need to check all the assumptions of equal varaince and normal distributions before I fit the model? I generally do when perfoming one way and two way ANOVA, but is it necessary for a DOE?

Thanks

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Oct 19, 2011 5:18 AM
(1067 views)

In general DOE data is analysed using regression models. The assumptions that you talk about are typically applicable to the residuals rather than the raw data, so in that sense you need to build the model first and then validate the assumptions. This can be done by inspecting the residuals. Bear in mind that with a DOE the quantities of data are usually small so it is sufficient to do this visually.

Dave

-Dave

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Nov 24, 2011 8:20 AM
(1067 views)

Thanks for this Dave. Im in the process of analysing my results. The response values from the DOE are not normally distributed, but when I screen my factors and build the model around the relevant ones, im saving the residuals of the reponse which when analysed, are normally distributed. Is this the correct way to go about validating the assumptions?

Cheers,

Chris

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Nov 25, 2011 11:15 AM
(1067 views)

Yes that's the correct way to go about it. You would not expect your response variable to have a normal distribution because you are peforming the DOE explicitly to induce changes in the response so that you can understand the influence of the predictor variables. However, once you have a model you are left with residuals which ideally should be normally distributed. I personally wouldn't worry hugely about this assumption (if you do then it is nolonger an assumption!) - you are unlikely to have enough data points to perform a test that refutes the assumption - however, studying the residuals can be useful to diagnose problems with a model or for areas of improvement.

-Dave

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Oct 20, 2011 6:16 AM
(1067 views)

Mixture DOE's are a unique type of DOE because the factors or parameters are interdependant. They behave differently than regular DOE's so I would suggest that you read a review by John Cornell on Mixture DOE to understand their unique statistical properties.

http://www.jstor.org/pss/1266850

The definitive book on Mixture Designs is available by John Cornell

http://www.amazon.com/Experiments-Mixtures-Analysis-Probability-Statistics/dp/0471393673

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Nov 8, 2011 3:31 AM
(1067 views)

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Nov 8, 2011 1:56 PM
(1067 views)