questions about cross-validation

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10-05-2011 09:50 AM
pengjia
New Contributor


I have a question about the pic above.

Below is the original descriptions attached to this pic.

"For example, the root-mean-squared prediction error may be smaller for a particular model. Therefore, you might conclude that it is the optimal model. However, when comparing to another model, the root-mean-squared prediction error may be closer to the average estimated prediction standard error. This is a more valid model, because when you predict at a point without data, you have only the estimated standard errors to assess your uncertainty of that prediction. When the average estimated prediction standard errors are close to the root-mean-squared prediction errors from cross-validation, you can be confident that the prediction standard errors are appropriate. In the figure above, both kriging models are good, but those at the left are slightly better."

My question is, that, the ones with the root-mean-squared prediction errors closer to average standard errors should be the right one, instead of the left one. Why does it say that those at the left are slightly better? Thanks.
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13 Replies
PhilMorefield
Occasional Contributor III
There's a typo in that pdf that I just noticed.  For average standard error, the formula is missing a square.  You can find the correct formulas here:

http://resources.arcgis.com/en/help/main/10.1/index.html#//00300000000z000000

"Average Standard Error" is the only formula that is different than you might expect.  It might be better called "Root-Mean-Variance."  We used this formula instead of a simple average because this formula is more directly comparable to the RMS.

Okay, I can see how they are similar. But would there be any reason to compare the "Average Standard Error" of a EBK model to RMSE of an IDW model, for example? It seems like looking at RMSE for both is a more defensible comparison.

Also, are there some common/likely reasons for seeing a RMS Standardized Prediction Error that is very large?
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EricKrause
Esri Regular Contributor
Yes, you would want to compare the RMSE between EBK and IDW. 

A large root-mean-square-standardized usually indicates the model is unstable.  The most common reason for this is because the Gaussian semivariogram is very unstable if the nugget is very small, compared to the sill.  Note that Stable with parameter=2 and K-Bessel with parameter=10 both correspond to the Gaussian semivariogram (it's a special case of both).

EDIT:  Oh, I understand what you were asking.  It doesn't make much sense to compare RMS and average standard error from different models, but it is useful to compare them within the same model because if the difference between them is large, it indicates that the model may have problems.
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PhilMorefield
Occasional Contributor III
Yes, you would want to compare the RMSE between EBK and IDW. 

A large root-mean-square-standardized usually indicates the model is unstable.  The most common reason for this is because the Gaussian semivariogram is very unstable if the nugget is very small, compared to the sill.  Note that Stable with parameter=2 and K-Bessel with parameter=10 both correspond to the Gaussian semivariogram (it's a special case of both).

EDIT:  Oh, I understand what you were asking.  It doesn't make much sense to compare RMS and average standard error from different models, but it is useful to compare them within the same model because if the difference between them is large, it indicates that the model may have problems.

Thanks Eric. I think I've got enough of an understanding to move forward with my project.
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SirajMazumder
New Contributor

Well, folks,

I have a similar query. What does the minus sign in the mean error and mean standardized error indicate? I mean, how does one interpret such a result?

Thanks

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