Problem 3.26, when assumptions are violated 1

Estimates of terms: We can estimate the mean response for Failure Time for problem 3.26 from the data by which is called the Predicted Value and which is called the Residual Value. 2

OK, what about when things go wrong Problem 3.26 data 3

Plot that data 4

Put in means and error bars 5

Plot Residuals vs. Predicted 6

A clear problem exists but run the test(s) for equal variances 7

What to do, what to do….. Use Box-Cox transformation to transform the data!!!!!!!!!!!!!! 8

Do the transformation and.. 9

Plot again 10

Residuals vs. Predicted 11

Testing equal variances 12

Now Normality Plot of Residuals 13

Test of Normality Goodness-of-Fit Test Shapiro-Wilk W Test W Prob<W Note: Ho = The data is from the Normal distribution. Small p-values reject Ho. 14

Whew, now do ANOVA…. 15

Compare Trt. groups with Tukey 16

Reporting results after the analysis of transformed data The purpose of diagnostics is to make sure that the assumptions are at least approximately correct, in which case the p-values reported for significance tests of means comparisons are valid. In reporting results, such as means of treatment groups, generally use the untransformed or raw data. Transformations of the data are used to obtain valid p- values, summaries of the original (untransformed or raw) data are used to report results. 17