ST 520 Statistical Principles of Clinical Trials - NCSU Statistics ...

CHAPTER 9 ST 520, A. TSIATIS and D. Zhang
Must be careful to choose θ, β, γ so that λ(t) ≥ 0 and if we also want a proper distribution, i.e.
S(t) → 0 as t → ∞ then
Λ(∞) = ∞.
Other popular distributions include the log normal distribution where
and the gamma distribution whose density
log(T) ∼ N(µ, σ 2 ),
f(t) is proportional to t ρ e −λt .
Remark: In most clinical trials applications and research in survival analysis it has become
common practice to use non-parametric and semi-parametric models where the shape of the
distribution function is left unspecified.
9.2 Censoring and Life-Table Methods
Two important issues in clinical trials where the primary endpoint of interest is time to an event
which are different than most studies:
1. Some individuals are still alive (event of interest has not occurred) at the time of analysis.
This results in right censored data.
2. The length of follow-up varies due to staggered entry.
This is illustrated in the schematic shown in the next page.
The time to event of interest in most clinical trials is the time from entry into the study until
death (right-hand panel).
In addition to censoring occurring because of insufficient follow-up, it may also occur for other
reasons such as
• loss to follow-up (patient drops out of the study, stops coming to the clinic or moves away)
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