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ZW
ZW
Occasional Contributor

Re: How do I determine statistical difference in slopes with non-normal data

Mark Bailey,

As long as I can state with 95% confidence using ANCOVA that my slopes are statistically different due to location or in this case ID I am very happy with the original model. I have included the age*age, age*age*age, and age*age*age*age just to be sure.ANCOVAOutput5.PNG

 

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Re: How do I determine statistical difference in slopes with non-normal data

I think that your conclusion is sound and defensible even with the slight pattern in the residual plot.

 

The new analysis with the higher orders should include ID alone and crossed with each of the Age terms. (Sorry I did not make that point clear before now.)

Learn it once, use it forever!
ZW
ZW
Occasional Contributor

Re: How do I determine statistical difference in slopes with non-normal data

Mark Bailey,

 

I included ID in this last model and received the error "Warning: Ill-conditioned regression problem" and the following output. ANCOVAOutput6.PNG

Thanks again for you help,

 

ZW

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Re: How do I determine statistical difference in slopes with non-normal data

I think that result is because not all of the ID levels have enough data to fit all the higher order terms.

Learn it once, use it forever!
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ZW
ZW
Occasional Contributor

Re: How do I determine statistical difference in slopes with non-normal data

Mark Bailey,

There are very few points for 2 of the individuals. If there is an alternative test please let me know. Thanks for all of your help!

ZW
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Re: How do I determine statistical difference in slopes with non-normal data

Right. I think that the best you can do is to use the reduced model (Age, ID, Age*ID) and the tests that the provide. (We already covered that option.)

Learn it once, use it forever!
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ZW
ZW
Occasional Contributor

Re: How do I determine statistical difference in slopes with non-normal data

Mark Bailey,
Perfect! I will proceed with this model. Thanks again!
ZW
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