Study questions

1. Write a summary of the results given in this case study in about 100 -150 words describing the difference in weaning weight among genotypes and the effects of the various covariates on weaning weight.

2. Fit the final statistical model again and save the residuals. Produce the boxplots by genotype and age of dam, as done earlier in this case study. How many outliers are now shown? What will you do about them?

3. Fit an additional cubic term (DC) for DAMAGE7 to the model and examine its statistical significance. Based on what you find would you include a cubic term or not?

4. We have observed the partial confounding between genotype and year. Run the model again without year and comment on the parameter estimates and their standard errors that you now find for genotype. Calculate the new value for the average standard error of difference between pairs of genotype means and compare with the earlier value.

5. Include BIRTHWT as a covariate in the model. What effect does this have on the analysis? Why? Using Genstat to calculate an analysis of variance to compare birth weights among genotypes may help to explain what has happened. Based on this do you think it appropriate to include BIRTHWT in the model and, if so, what interpretation do you put on the results?