Natural history of disese Chap - 6

Question 1

Why is person year approach not good to calculate survival rate?

Answer 1

Those lost to follow-up and remaining alive at the** study terminatio**n are included in the denominator reducing the mortality rate.

Question 2

Two main assumption in Kaplan-Meyer Survival Analysis

Answer 2

1. Does not group the times in which outcomes occur into intervals
   * Each row is defined by the time at which the next sbj experiences the outcome (death)
2. Withdrawn sbj are assumed to be at risk for the outcome up to and including the time they are withdrawn

Note: Kaplan-Meyer Survival Analysis includes the dead individuals as the number at risk after the death occurs.

Question 3

Acturial survival analysis (life - table) assumptions (5)

Answer 3

1. Clear indication of time-zero i.e. time of first exposure ( when did the patient first get exposed, when did the biological onset of disease happen? but this is difficult to do ; date of treatment is usually taken as zero time)
2. Well defined study outcome e.g. what is the outcome of the disease is it death, or chronic disease or first occurance.
3. Lost to follow-up are assumed to have identical study outcome
4. Risk of outcome is independent of calender time i.e. identical prognosis if the subject enters study early or late
5. Risk of outcome remain constant within the intervals used to construct the life table

Question 4

What are the various wasy of expressing prognosis (likely course of the disease)?

Answer 4

1. **Case Fatality **= number of ppl who die of the disease/number of people who have the disease ; short- term , acute condition
2. Person - Years = number of
   deaths divided by the person-years over which a group is observed [**limitation: **risk of outcome is believed to be the same in any person year observed]
3. **Five year survival = percentage of patients who are alive 5 years after treatment begins or 5 years after diagnosis. [limitation: **if we want to look at the survival experience of a group of patients who were diagnosed less than 5 years ago, we clearly cannot use this criterion]
4. Life-table analysis [Acturial and Kaplan- Mier] = most accurate as it takes in all information in the table
5. Relative Survival = Observed survival in people with disease/expected survival if disease were absent [We would want to compare the observed survival in 80-year-old men with colorectal cancer to
   the expected survival of 80-year-old men without colorectal cancer]
6. **Median Survival **= defined as the length of time that half (50%) of the study population survives.