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What assumptions did you make? 13 (f) (Advanced) Penrose, Nelson, and Fisher (1985) built their regression model using abdominal exercises just the first 143 cases of the 252 cases in the dataset. The remaining 109 cases were used to get a true estimate of the error of the model (c.f. question d). Here is an alternative "cross-validation" approach to error estimation I would 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) above, refit the model coefficients using all the cases but the abdominal exercises first abdominal exercises 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.)
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