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| rope, oxygen, antioxidants, sustainablefisheries, repeatability, fatskin, ediet, inversion, shoppingforvitamins, cytodyne, senior centers, | The remaining 109 cases were used to get a true estimate of the error of the model (c.f. question nutros d). Here is an alternative "cross-validation" approach to error estimation I would nutros like you to consider (but do not actually perform): i. Build a regression model using all 252 cases. ii. Using the model (i.e., variables) nutros above, refit the model coefficients using all the cases but the first and record the error in body fat using this model on the first case. iii. Repeat ii., leaving out, in turn, just the second, just the third, ... , just the 252nd case, each time recording the error. iv. Compute the standard deviation of the 252 errors to provide an estimate of the accuracy of the model. 14 Why is the resulting error estimate (the standard deviation in iv.) a better estimate of the true error that one should expect in using the model than the standard error? |
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| (Note: 1 inch = 2.54 cm.) Comment: Most students see that the model fatskin should not be fatskin used for women; other students get into more subtle issues regarding the age of college students compared to the age of the men in the dataset. 11 (d) Comment on the accuracy of your model. Discuss, in particular, what the standard error means. What kind of error should a user of this model expect as opposed to the fatskin prediction error for those folks who were used to build the model? Comment: Before starting this lab I have already discussed the "incestuous" nature of the standard error value and how the model is likely to give an error larger than the standard error for cases that were not used to build it. 12 (e) Estimate the percentage of American men with percentage of body fat less than 15% (the maximum for good health given by Bailey (1991) above). What assumptions did you make? 13 (f) (Advanced) Penrose, Nelson, and Fisher (1985) built their regression model using just the first 143 cases of the 252 cases in the dataset. |
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