Download Homework 4

Biostatistics 514/517
Autumn 2013
Homework 4
Reminder: homework questions should be answered in complete sentences. This is part of
making “good faith effort” and necessary for getting credit for the assignment.
1. In Lecture 6, consider the “Motivating Example” that starts on slide #26.
a. What would the data look like in the standard notation for right-censored survival data?
Complete the last column of the following table:
T
1
1
2
2
3
3
D
n
1
0
1
0
1
0
b. Either by brute force, or by using a bit of programming, get the data into STATA (or your
preferred software), and make a Kaplan-Meier curve. Compare your results to the calculations
on slide #38.
2. A study compared the success rates of two different procedures for removing kidney stones:
open surgery and percutaneous nephrolithotomy (PCNL), a minimally invasive technique. Here
are the numbers of procedures that were successful or not at getting rid of patients’ kidney
stones, by type of procedure. The data are separated by patients with small kidney stones and
large kidney stones:
Patients with Small Stones:
Open
PCNL
Surgery
Success
81
234
Failure
6
36
Patients with Large Stones:
Open
PCNL
Surgery
Success
192
55
Failure
71
25
a. Combining the data for small and large stones, find the percent of kidney stones that were
successfully removed for each of the two medical procedures.
b. What percent of small kidney stones were successfully removed? What percent of all large
kidney stones were successfully removed? Which type of kidney stones appears to be easier to
treat?
c. Find the percent of successful procedures of each type for small kidney stones only. Do the
same for large kidney stones. Compare your results with a. Do you see a paradox? Explain
carefully, referring to the data, how this paradox can happen.
d. The predictor of interest is type of procedure. The outcome of interest is success/failure.
What is the role of the type of stone (small vs. large) in this study? Confounder, precision
variable, effect modifier?
3. Using the WCGS dataset, make a scatterplot of SBP versus weight and add a loess line or
similar scatterplot smooth. Make, and plot, a smooth for at least 3 different “bandwidths,” or
vary the amount of smoothing in some other way. How do they compare? Which is most
useful for judging the linearity or lack of linearity of the relationship?
4. The timing of various stages in the contraction of the heart, determined by
electrocardiogram (EKG) can be used to diagnose heart problems. A commonly measured time
interval in the contraction of the ventricles is called the QRS wave. A study was conducted to
see if longer QRS times were related to the ability to induces rapid heart rhythms (called
inducible ventricular tachycardia or IVT), which have been associated with adverse outcomes.
Use the QRS dataset on the course website to complete the following.
a. Display the data numerically to help understand whether QRS time is related to IVT.
b. Display the data graphically to help understand whether QRS time is related to IVT.
c. QRS time is conventionally considered abnormal if the value is greater than 120 ms.
Generate a numeric display to help understand if abnormal QRS is related to IVT.
d. What are the advantages and disadvantages of treating QRS as binary instead of continuous?
Read the article CMFVP: Rivkin SE, Green SJ, Metch BJ et al. (1993) One versus 2 years of
CMFVP adjuvant chemotherapy in axillary node-positive and estrogen receptor-negative
patients: a Southwest Oncology Group Study. J. Clinical Oncology 11: 1710-1716, available
online (you may need to use a UW computer). The next three questions pertain to this article
as well as the CARET and CAST articles from HW1.
5. For each of the studies CARET, CAST and CMFVP, was the treatment assignment blinded to
participants? Was it blinded to investigators? Why or why not?
6. Did the investigators include the correct patients in the analysis in the CMFVP study?
Why or why not?
7. How might you have modified the design of the CMFVP study to avoid some of the problems
associated with non-compliance to treatment? Would your modified design address exactly the
same questions as the CMFVP study did?