Stab22 Midterm Examination Solutions - University Of Toronto Scarborough, 2007 Page 10

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Do you see any problems in this plot? Explain briefly (1 sentence).
Yes: I see a steady increase in residuals between 1979 and 1986, and
a steady decrease after that. (The residual plot should show a random,
formless scatter of points, so any patterns you can describe in a few
words are a problem. I think this is a bigger problem than the two large
residuals in 1976 and 1977.)
17. (7 marks) Breast cancer that is detected in its early stages can be treated. In the past,
the preferred treatment was mastectomy (removal of the breast); now, it is usual to
remove the tumour, and have the patient undergo radiation. A medical team compares
the survival times after surgery of all women who have had either treatment.
(a) What is the explanatory variable here?
Treatment: that is, mastectomy or removal of tumour plus radiation.
(b) What is the response variable here?
Survival time.
(c) Is this study (choose one):
i. * an observational study
ii. an experiment?
(d) Will the medical team be able to conclude that the better of the two treatments
causes a longer average survival time (circle your preferred answer)?
i. yes
ii. * no (an observational study won’t let you conclude cause and effect, not at
least without extra work. If you thought it was an experiment, I gave you
a point for answering “yes”, on the grounds that an experiment can let you
infer cause and effect.
18. (5 marks) A statistical experiment is to be done to compare three treatments. 9
subjects are available, named: Alomar, Bikalis, Cranston, Durr, George, Han, Imrani,
10

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