EPIDEMIOLOGY SURVIVAL ANALYSIS

Running head: EPIDEMIOLOGY SURVIVAL ANALYSIS 1
Epidemiology Survival Analysis
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EPIDEMIOLOGY SURVIVAL ANALYSIS 2
Question 1.
A prospective study with a 2-year (24-month) follow-up was conducted. Results are shown in the table for
individuals who either died or were censored before the end of the follow-up period. (75 marks)
Follow up time, in months Event
2 Death
4 Censored
7 Censored
8 Death
12 Censored
15 Death
17 Death
19 Death
20 Censored
23 Death
1. Using the data from the table, for all deaths calculate (1) the probability of death at the exact time
when each death occurred, (2) the probability of survival beyond the time when each death
occurred, and (3) the cumulative probabilities of survival.
2. What is the cumulative survival probability at the end of the follow-up period?
3. What is the simple proportion of individuals apparently surviving (i.e., not observed to die) through
the end of the study’s observation period?
4. Why are the simple proportion surviving and the cumulative probability of survival different?
5. Using the same data, calculate the overall death rate per 100 person-years. (To facilitate your
calculations, you may wish to calculate the number of person-months and then convert that to the
number of person-years.)
6. Calculate the rates separately for the first and second years of follow-up. (For this calculation,
assume that the individual who withdrew at month 12 withdrew just after midnight on the last day
of the month.)
7. Assuming that there was no random variability, was it appropriate to calculate the rate per
person-year for the total 2-year duration of follow-up?
8. What is the most important assumption underlying the use of both survival analysis and the
person-time approach?
9. Now, assume that the length of follow-up was the same for all individuals (except those who
died). Calculate the proportion of deaths and the odds of death in this cohort.
10. Why are these figures so different in this study?
Question 2.
The baseline point prevalence of hypertension in African American women aged 45 to 64 years included
in the Atherosclerosis Risk in Communities (ARIC) cohort study (Ref 1) was found to be 56%. In this
study, over a follow-up period of 6 years, the average yearly incidence of hypertension in African
American women was estimated to be about 5% and stable over the years(Ref 2). Using these data,
estimate the average duration of hypertension in African American women in the ARIC Study. (25 marks)

EPIDEMIOLOGY SURVIVAL ANALYSIS 3
Epidemiology Survival Analysis
Question 1
1. Using the data from the table we are able to calculate the following
(1) The probability of death at the exact time when each death occurred (P(Death)) is the
total number dead up to that time divided by the number of participants (3).
P ( Death )= Number Dead
Number of participants
(2) The probability of survival beyond the time when each death occurred, (P(Survive))
is one minus the probability of death.
P ( Survive )=1−P ( Death )
(3) The cumulative probabilities of survival (cumulative P(survive)) is the probability
surviving in during the previous death multiplied by the probability during the current
death.
cumulative P ( survive )=P ( Survive ) previous death x P ( Survive ) current death
The results are summarized in table below.
Follow Up
Time (Month)
Event P(Death) P(Survive) Cumulative
P(Survive)
2 Death 1
10 =0.100 0.900 0.900