Presentation
at a Training Program on Biostatistics for physician managers working in Public
Health Administration, Qassim Province on May 1, 2013 by Professor Omar Hasan
Kasule Sr MB ChB (MUK), MPH (Harvard), DrPH (Harvard) EM: omarkasule@yahoo.com
INTRODUCTION
TO SURVIVAL ANALYSIS
- Survival analysis is used to study survival duration and the effects of covariates on survival.
- Time is measured as time to relapse, length of remission, remission duration, survival after relapse, time to death, or time to a complication.
- The best zero time is point of randomization. Other zero times are: enrolment, the first visit, first symptoms, diagnosis, and start of treatment.
PROBLEMS OF SURVIVAL
ANALYSIS
- Problems of survival analysis are censoring, truncation, and competing causes of death.
- Censoring is loss of information due to withdrawal from the study, study termination, loss to follow-up, or death due to a competing risk. Clinical trials analysis based on the intention to treat is more conservative than censored analysis.
- In left truncation, only individuals who survive a certain time are included in the sample. In right truncation only individuals who have experienced the event of interest by a given time are included in the sample.
- Competing causes of death are one cause of censoring that bias survival estimates.
NON-REGRESSION
SURVIVAL ANALYSIS
- Two non-regression methods are used in survival analysis: the life-table and the Kaplan-Maier methods.
- The life-table methods better with large data sets and when the time of occurrence of an event cannot be measured precisely. It leads to bias by assuming that withdrawals occur at the start of the interval when in reality they occur throughout the interval.
- The Kaplan-Maier method is best used for small data sets in which the time of event occurrence is measured precisely. It is an improvement on the life-table method in the handling of withdrawals. The assumption could therefore create bias or imprecision. The Kaplan-Maier method avoids this complication by not fixing the time intervals in advance.
REGRESSION METHODS FOR SURVIVAL ANALYSIS
- The Proportional hazards, a semi-parametric method proposed by Sir David Cox in 1972, is the most popular regression method for survival analysis.
- It is used on data whose distribution is unknown.