Typical Survival Data Arising From a Clinical Trial. Censoring. The Survivor Function. Mathematical Definitions Introduction


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1 Outline CHL 5225H Advanced Statistical Methods for Clinical Trials: Survival Analysis Prof. Kevin E. Thorpe Defining Survival Data Mathematical Definitions Nonparametric Estimates of Survival Comparing Survival Dalla Lana School of Public Health University of Toronto Measuring Treatment Effect Parametric Estimates of Survival References Defining Survival Data Introduction Definition of Failure Time Interest centres on patients who are at risk for some event and the time to the occurrence of the event. s: Time to death Time to stroke Time to disease recurrence Three requirements to precisely determine failure time: unambiguous time origin scale for measuring the passage of time meaning of the failure event must be entirely clear
2 Censoring Typical Survival Data Arising From a Clinical Trial Patients who are not observed to failure are called censored Some Causes: study concludes before all patients destined to fail have actually failed patient withdraws from study before failure observed Mathematical Definitions Introduction The Survivor Function The survivor function F(t) is defined to be Let the failure time be represented by a nonnegative random variable T. Usually, we consider continuous distributions. Discrete and mixed discretecontinuous distributions can also handled. F(t) = Pr(T t) It is sometimes useful to consider the cumulative risk of an event. The cumulative risk function is defined by R(t) = 1 F(t) Note that the definition of F(t) leads to left continuous CDF rather than the usual right continuity.
3 Probability Density Function Hazard Function Continuous Distributions Continuous T. which gives f (t) = F (t) = F(t) = Pr(t T < t + ) lim 0+ t f (u)du Discrete T. Usually handled by assigning to the density a component f j δ(t a j ) for an atom f j at a j where δ( ) is the Dirac delta function. The hazard function (also called agespecific failure rate or force of mortality is defined by h(t) = Pr(t T < t + t T ) lim 0+ By the definition of conditional probability h(t) = f (t)/f(t) Sometimes the hazard is denoted by λ(t). Hazard Function Continuous Distributions (continued) By the definitions of f (t) and that F(0) = 1 we have and h(t) = F (t)/f(t) = d log F(t)/dt ( t ) F(t) = exp h(u)du 0 = exp[ H(t)] Hazard Function Discrete Distributions If there is an atom f j of probability at time a j then h j = f j /F(a j ) = f j /(f j + f j+1 + ) With this definition it can be shown that F(t) = a j <t(1 h j ) By defining the integrated hazard as where H( ) is called the integrated hazard. Finally, f (t) = h(t)exp[ H(t)] we still have H(t) = log(1 h j ) a j <t F(t) = exp[ H(t)]
4 The Likelihood Function Survival Models We assume each patient has a failure time t i and a censoring time c i associated with them. In practice, we observe x i = min(t i, c i ). L = u f (t i; φ) c F(c i; φ) l = u log f (t i; φ) + c log F(c i; φ) l = u log h(x i; φ) + log F(x i ; φ) Suppose in addition to survival some covariates z are measured with the intent of modeling their association with survival. For some baseline hazard function h 0 (t) (or survivor function F 0 (t) we have the following commonly used models. The Accelerated Life model is defined by F(t; z) = F 0 [tψ(z)] or h(t; z) = h 0 [tψ(z)]ψ(z) The choice of F 0 is dictated by a choice of parametric model. The Proportional Hazard model is defined by F(t; z) = [F 0 (t)] ψ(z) or h(t; z) = ψ(z)h 0 (t) The baseline hazard does not need to be explicitly known for this model. The Likelihood Function We begin by assuming a discrete distribution atoms of probability at g unique failure times t 1, t 2,..., t g. It can be shown that the loglikelihood is l = j [d j log h j + (r j d j ) log(1 h j )] where r j is the number of patients in view at time t j and d j is the number who fail at t j. By convention, r j includes individuals who are censored at t j. This is identical to the log likelihood for g independent binomials with r j trials, d j failures and probability of failure h j. This fact is used in the derivation of the results on the next slide. ProductLimit or KaplanMeier Estimate Suppose g unique failure times are observed. Put them in order from smallest to largest. t 1, t 2,..., t g The survivor function is estimated at time t j by ˆF(t j ) = j i=1 ( 1 d ) i = r i j ( ) ri d i Note the similarity between the first expression above and the expression for a discrete survivor function, F(t) = a j <t (1 h j) given previously. The variance of ˆF(t) at time t j is (Greenwood s formula) V ( ˆF(t j )) = [ ˆF(t j )] 2 j i=1 i=1 r i d i r i (r i d i )
5 Suppose we had the following data (+ indicates censored observations). [1] [12] [23] t j r j d j P(t j ) = 1 d j /r j ˆF(t j ) SE /27 = = /25 = = /21 = = /19 = = /17 = = /13 = = /12 = = /11 = = /10 = = /6 = = /4 = = Proportion Event Free Kaplan Meier Survivor Function with Greenwood 95% Confidence Interval R Code > library(survival) > eg1a.km < survfit(surv(days, status), data = eg1a) > summary(eg1a.km) > plot(eg1a.km, mark.time = FALSE, xlab = "Days", + ylab = "Proportion EventFree", + main = "KaplanMeier Survivor Function\n with Greenwood 95% Confidence Interval") Days
6 Actuarial Estimator Comparing Survival Suppose instead of exact failure/censoring times we know only the numbers of failures and censored observations between time intervals. For each interval (t j 1, t j ] we define r j 1 to be the number at risk at t j 1, d j to be the number who fail during the interval and m j to be the number censored during the interval. Define r j = r j m j. Then, the actuarial estimator of survival is F(t j ) = ( 1 d ) k r k j k There are a variety of methods that can be used to compare survival in various ways. Point in time comparison (two groups) MantelHaenszel or Log Rank Test (two or more survival curves) Modelbased tests (two or more groups) Point in Time Comparison The object is to compare survival between two treatments at a prespecified time point t 0. From the KaplanMeier procedure and Greenwood s formula we have, Treatment : ˆF T (t 0 ) and V [ ˆF T (t 0 )] Control : ˆF C (t 0 ) and V [ ˆF C (t 0 )] Then, to conduct an approximate test of the null hypothesis H 0 : F T (t 0 ) = F C (t 0 ) we calculate the Z statistic, Z = Under H 0, Z N(0, 1). ˆF T (t 0 ) ˆF C (t 0 ) V [ ˆF T (t 0 )] + V [ ˆF C (t 0 )] MantelHaenszel Test Consider the two survival curves as a series of 2x2 contingency tables constructed at each time point where one or more events occur. At a time point t j we have a table of the form: Treatment Control Total Dead d Tj d Cj d j Alive n Tj d Tj n Cj d Cj N j d j Total n Tj n Cj N j Then, from the hypergeometric distribution, E(d Tj ) = n T j d j N j V (d Tj ) = n T j n Cj (N j d j )d j N 2 j (N j 1)
7 MantelHaenszel Test Continued Comparing Survival in R Define the following quantities. O T = d Tj E T = E(d Tj ) O C = d j O T E C = d j E T V = V (d Tj ) Now, the null hypothesis H 0 : F T (t) = F C (t) is usually tested with a χ 2 (1) test, but may also be tested with a Z test. and χ 2 = (O T E T ) 2 V Z = O T E T V In R the survdiff() function is used to compare survival curves. This performs tests in a family described by Harrington and Fleming (Biometrika: 1982) where the weights on each event time are F(t) ρ. The survdiff() function accepts an argument rho. If rho = 0 (the default) you get the MantelHaenszel test and if rho = 1 you get the Peto & Peto modification of the GehanWilcoxon test. This is often called the Log Rank Test. Proportion Event Free > survdiff(surv(days, status) ~ rx, data = eg1b) Call: survdiff(formula = Surv(days, status) ~ rx, data = eg1b) N Observed Expected (OE)^2/E (OE)^2/V rx=control rx=treatment Chisq= 0.9 on 1 degrees of freedom, p= Days
8 Measuring the Treatment Effect Among other things, clinical trials are intended to provide an estimate of the treatment effect. For a survival outcome, two types of estimates are commonly used. Point in time Risk Ratio or Risk Reduction. All time or whole curve effect measures by means of Hazard Ratios. Point in Time Treatment Effect Suppose a trial is comparing a treatment T to a control C. The usual expectation would be that the risk of the event would be lower in the treatment group than the control group. There may be situations where there is some clinically meaningful time point t 0 say, that is of interest. Consider three point in time estimates of risk reduction. Risk Difference = R C (t 0 ) R T (t 0 ) Risk Ratio = R T (t 0 )/R C (t 0 ) Relative Risk Reduction = R C (t 0 ) R T (t 0 ) R C (t 0 ) = 1 R T (t 0 ) R C (t 0 ) Estimating Risk Difference Estimate T & C survival curves using KaplanMeier procedure and compute. RD = ˆR C (t 0 ) ˆR T (t 0 ) = (1 ˆF C (t 0 )) (1 ˆF T (t 0 )) = ˆF T (t 0 ) ˆF C (t 0 ) Calculate V ( ˆF T (t 0 )) and V ( ˆF C (t 0 )) using Greenwood s formula (ie. SE 2 ). Then, 95% CI on the RD is (approximately): RD ± 1.96 V ( ˆF T (t)) + V ( ˆF C (t)) Estimating Risk Ratio Estimate T & C survival curves using KaplanMeier procedure and compute. RR = ˆR T (t 0 )/ˆR C (t 0 ) = 1 ˆF T (t 0 ) 1 ˆF C (t 0 ) Calculate V (log ˆR T (t 0 )) and V (log ˆR C (t 0 )) using the following formula. V (log ˆR(t 0 )) = V ( ˆF(t 0 )) (ˆR(t 0 )) 2 Then, 95% CI on RR is (approximately): RR e ±1.96 V (log ˆR T (t 0 ))+V (log ˆR C (t 0 ))
9 Estimating Relative Risk Reduction Estimating Hazard Ratio Estimate T & C survival curves using KaplanMeier procedure and compute. RRR = 1 ˆR T (t 0 ) ˆR C (t 0 ) = 1 RR Determine the CI for RR. Say this is (RR L, RR U ). The the CI for RRR is (1 RR U, 1 RR L ). The MantelHaenzsel test calculations enable estimation of the hazard ratio. ˆψ = O T /E T O C /E C V [log ˆψ] = E T E C Various modelbased estimates are preferred where appropriate. Introduction to Parametric Survival Distributions The Exponential Distribution We saw earlier that the KaplanMeier estimate of survival is not particularly smooth. By assuming a particular parametric form for the survival distribution, smooth estimates of survival can be obtained. Some commonly used distributions in survival analysis are: Exponential Weibull Lognormal Loglogistic We will look at the exponential and Weibull distributions in more detail. For the exponential distribution with parameter ρ and mean 1/ρ, we have F(t) = e ρt, f (t) = ρe ρt, h(t) = ρ, H(t) = ρt A plot of H(t) = log( ˆF(t)), where ˆF(t) is the KaplanMeier estimate versus t will be linear if the exponential distribution is appropriate. The MLE for ρ is ˆρ = d/ x i where d is the total number of failures.
10 The Exponential Likelihood Likelihood Ratio Based Confidence Interval For exponential failure times with d failures we get l(ρ; x) = u log h(x i; φ) + log F(x i ; φ) = u log ρ ρ x i = d log ρ ρ x i U(ρ) = d/ρ x i I (ρ) = d/ρ 2 It is easy to show that solving U(ρ) = 0 gives ˆρ = d/ x i as the MLE and 1/I (ˆρ) is an estimate of its standard error, although not the best to use in practice. Confidence intervals based on the likelihood ratio statistic or from a fitted model are preferred. The likelihood ratio statistic for the null hypothesis ρ = ρ 0 is W (ρ 0 ) = W = 2[l(ˆρ) l(ρ 0 )] This has a nice simple form in the exponential case and is distributed approximately as a chisquared variate with 1 degree of freedom. A 1 α confidence region is {ρ : W (ρ) c 1,α} where c1,α is the upper α point of the chisquared distribution with 1 degree of freedom. Cumulative Hazard Plot Direct computation of hazard and symmetric 95% CI. > rhohat < sum(eg1a$status)/sum(eg1a$days) > se.rhohat < sqrt(rhohat^2/sum(eg1a$status)) > symci < rhohat + c(1, 1) * qnorm(0.975) * se.rhohat > c(rhohat, symci) [1] H(t) Time
11 log likelihood Exponential Log Likelihood Model based estimate. > eg1a.sr < survreg(surv(days, status) ~ 1, data = eg1a, + dist = "exponential") > summary(eg1a.sr) Call: survreg(formula = Surv(days, status) ~ 1, data = eg1a, dist = "exponenti Value Std. Error z p (Intercept) e63 Scale fixed at 1 Exponential distribution Loglik(model)= 70 Loglik(intercept only)= 70 Number of NewtonRaphson Iterations: 5 n= ρ > exp(coef(eg1a.sr)) (Intercept) The Two Group Problem Comparing confidence intervals: Symmetric: Likelihood Based: Model Based: Suppose you have exponentially distributed failure data for a treatment group T and a control group C. Estimate ˆρ T = d T / x Ti, ˆρ C = d C / x Ci and ˆψ = ˆρ T /ˆρ C. It turns out that V (log ˆψ) 1/d T + 1/d C which permits approximate Ztests and confidence intervals to be calculated.
12 Direct Calculation > rhohats < tapply(eg1b$status, eg1b$rx, sum)/tapply(eg1b$days, + eg1b$rx, sum) > hr < rhohats[2]/rhohats[1] > lhrse < sqrt(sum(1/tapply(eg1b$status, eg1b$rx, + sum))) > c(hr, log(hr), lhrse) Treatment Treatment Model based. > eg1b.sr < survreg(surv(days, status) ~ rx, data = eg1b, + dist = "exponential") > summary(eg1b.sr) Call: survreg(formula = Surv(days, status) ~ rx, data = eg1b, dist = "exponent Value Std. Error z p (Intercept) e63 rxtreatment e01 Scale fixed at 1 Exponential distribution Loglik(model)= Loglik(intercept only)= Chisq= 0.71 on 1 degrees of freedom, p= 0.4 Number of NewtonRaphson Iterations: 5 n= 54 The Weibull Distribution The Weibull distribution has the following: Model based (continued). > exp(coef(eg1b.sr)[2]) rxtreatment F(t) = exp[ (ρt) κ ] f (t) = κρ(ρt) κ 1 exp[ (ρt) κ ] h(t) = κρ(ρt) κ 1 Note that if κ = 1 we are left with an exponential distribution with a hazard of ρ. A plot of log( log( ˆF(t))) versus log(t) will be linear if the Weibull distribution is appropriate. The biggest difficulty is that the coefficient estimates don t have nice straight forward interpretations like other models we ve used. A scale parameter is estimated and e β i /scale gives a hazard ratio.
13 Fitting a Weibull in R log(h(t) log(t) > eg2b.sr < survreg(surv(days, status) ~ rx, data = eg1b, + dist = "weibull") > summary(eg2b.sr) Call: survreg(formula = Surv(days, status) ~ rx, data = eg1b, dist = "weibull" Value Std. Error z p (Intercept) e176 rxtreatment e01 Log(scale) e03 Scale= Weibull distribution Loglik(model)= Loglik(intercept only)= Chisq= 0.83 on 1 degrees of freedom, p= 0.36 Number of NewtonRaphson Iterations: 8 n= 54 The hazard ratio for treatment is given by: > exp(coef(eg2b.sr)[2]/eg2b.sr$scale) rxtreatment Survival Days
14 Loglogistic and Lognormal Distributions Computing log hazard in R These distributions are very similar. Both result in nonproportional hazards models. If log T has a logistic distribution, we say T is loglogistic If log T as a normal distribtion, we say T is lognormal If log h(t) is nonmonotonic in t then these distributions are possible. > kmf < survfit(surv(days, status) ~ rx, data = eg1b) > H1 < log(kmf[1]$surv) > H2 < log(kmf[2]$surv) > Ht1 < eg1b.km[1]$time > Ht2 < eg1b.km[2]$time > dh1 < diff(h1)/diff(ht1) > dh2 < diff(h2)/diff(ht2) > loghaz < data.frame(ldh = log(c(dh1, dh2)), times = c(ht1[1], + Ht2[1]), rx = factor(c(rep(1, length(dh1)), + rep(2, length(dh2))), labels = c("control", + "Treatment"))) 2 Plotting log hazard in R > library(lattice) > xyplot(ldh ~ times, data = loghaz, groups = rx, + type = c("p", "smooth"), ylab = "log hazard", + xlab = "Time") log hazard Time
15 Fitting a loglogistic Model in R > llog.fit < survreg(surv(days, status) ~ rx, data = eg1b, + dist = "loglogistic") > summary(llog.fit) Call: > pct < 0:99/100 survreg(formula = Surv(days, status) ~ rx, data = eg1b, dist = "loglogistic") > llog.pred < predict(llog.fit, newdata = data.frame(rx = factor(1:2, Value Std. Error z p + labels = c("control", "Treatment"))), type = "quantile", (Intercept) e p = pct) rxtreatment e01 > plot(kmf, mark.time = FALSE, lty = 1:2) Log(scale) e05 > matlines(t(llog.pred), cbind(1  pct, 1  pct), + col = "red", lty = 1:2) Scale= 0.48 Log logistic distribution Loglik(model)= Loglik(intercept only)= Chisq= 0.71 on 1 degrees of freedom, p= 0.4 Number of NewtonRaphson Iterations: 5 n= 54 Plotting the Result Adding Weibull > weib.pred < predict(eg2b.sr,, newdata = data.frame(rx = factor(1:2, + labels = c("control", "Treatment"))), type = "quantile", + p = pct) > matlines(t(weib.pred), cbind(1  pct, 1  pct), + col = "blue", lty = 1:2)
16 References Cox D.R. and Oakes D. (1984). Analysis of Survival Data. Chapman and Hall Kalbfleisch J.D. and Prentice R.L. (1980). The Statistical Analysis of Failure Time Data. New York: Wiley
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