# 5 Modeling Survival Data with Parametric Regression

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1 5 Modeling Survival Data with Parametric Regression Models 5. The Accelerated Failure Time Model Before talking about parametric regression models for survival data, let us introduce the accelerated failure time (AFT) Model. Denote by S (t) ands 2 (t) the survival functions of two populations. The AFT models says that there is a constant c>0 such that S (t) =S 2 (ct) for all t 0. (5.) This model implies that the aging rate of population is c times as much as that of population 2. (For example, if S (t) is the survival function for the dog population and S 2 (t) isthesurvival function for the human population, then the conventional wisdom that a year for a dog is equivalent to 7 years for a human implies c =7,andS (t) =S 2 (7t). So the probability that a dog can survive 0 years or beyond is the same as the probability that a human subject can survive 70 years or beyond) Let µ i be the mean survival time for population i and let ϕ i be the population quantiles such that S i (t)(ϕ i )=θ for some θ (0, ). Then µ 2 = = c = c S 2 (t)dt S 2 (cu)du (t = cu) S (u)du = cµ and S 2 (ϕ 2 )=θ = S (ϕ )=S 2 (cϕ ). Assume that S 2 (t) is a strictly decreasing function. Then we have ϕ 2 = cϕ. PAGE 00

2 This simple argument tells us that under the accelerated failure time model (5.), the expected survival time, median survival time of population 2 all are c times as much as those of population. Suppose we have a sample of size n from a target population. For subject i (i =, 2,..., n), we have observed values of covariates z i,z i2,..., z ip and possibly censored survival time T i.the procedure Proc Lifereg in SAS fits models to data specified by the following equations log(t i )=β 0 + β z i β p z ip + σε i, (5.2) where β 0,..., β p are the regression coefficients of interest, σ is a scale parameter and ε i are the random disturbance terms, usually assumed to be independent and identically distributed with some density function f(ε). The reason why we take logarithm of T i is obvious considering the fact that the survival times are always positive (with probability ). Equation (5.2) is very similar to a linear regression model for the log-transformed response variable Y i =log(t i ). In a linear regression, the random error term e i is usually assumed to be i.i.d. from N(0,σ 2 )sothate i can be written as e i = σε i,whereε i are i.i.d. from N(0, ) in this case. At this moment, let us see how the regression coefficients in model (5.2) can be interpreted in general. We will investigate their interpretation more closely later when we consider more specific models (i.e., with different distributional assumptions for ε i ). For this purpose, let us consider β k (k =,,p). Holding other covariate values fixed, let us increase covariate z k by one unit from z k to z k + and denote by T and T 2 the corresponding survival times for the two populations with covariate values z k and z k + (with other covariate values fixed). Then T and T 2 canbeexpressedas T = e β 0+β z +...+β k z k +...+β pz p e σε = c e σε T 2 = e β 0+β z +...+β k (z k +)+...+β pz p e σε 2 = c 2 e σε 2 where c 2 and c are two constants related by c 2 = c e β k. The corresponding survival functions PAGE 0

3 are S (t) = P [T t] =P [c e σε t] =P [e σε c t], S 2 (t) = P [T 2 t] =P [c 2 e σε 2 t] =P [e σε 2 c 2 t] Since ε and ε 2 have the same distribution, and c 2 = c e β k,wehave S 2 (e β k t)=p [e σε 2 c 2 e β k t]=p [e σε 2 c e β k e β k t]=p [e σε 2 t] =P [e σε t] =S (t). Therefore, we have accelerated failure time model between populations (covariate value=z k ) and 2 (covariate value=z k +)withc =e β k. So if we increase the covariate value of zk by one unit while holding other covariate values unchanged, the corresponding average survival time µ 2 and µ will be related by µ 2 =e β k µ. If β k is small, then µ 2 µ µ =e β k β k. Similarly we have for the population quantiles ϕ i ϕ 2 ϕ ϕ =e β k β k. Therefore, when β k is small, it can interpreted as the percentage increase if β k > 0 or percentage decrease if β k < 0 in the average survival time and/or median survival time when we increase the covariate value of z k by one unit. Thus the greater value of the covariate with positive β k is more beneficial in improving survival time for the target population. This interpretation of β k is very similar to that in a linear regression model. 5.2 Some Popular AFT Models We can assume different distributions for the disturbance term ε i in model (5.2). For example, we can assume ε i i.i.d. N(0, ). This assumption is equivalent to assuming that T i has log-normal PAGE 02

4 distribution (of course, conditional on the covariates z s). In this section, we will introduce some popular parametric models for T i (equivalently for ε i ). The following table gives some of these distributions: Distribution of ε Distribution of T Syntax in Proc Lifereg extreme values (2 par.) Weibull dist = weibull extreme values ( par.) exponential dist = exponential log-gamma gamma dist = gamma logistic log-logistic dist = llogistic normal log-normal dist = lnormal In Proc Lifereg of SAS, all models are named for the distribution of T rather than the distribution of ε. Although these above models fitted by Proc Lifereg all are AFT models (so the regression coefficients have a unified interpretation), different distributions assume different shapes for the hazard function. The exponential model The simplest model is the exponential model where T at z = 0 (usually referred to as the baseline) has exponential distribution with constant hazard exp( β 0 ). This is equivalent to assuming that σ = and ε has a standard extreme value distribution f(ε) =e ε eε, which has the density function shown in Figure 5.. (So e ε has the standard exponential distribution with constant hazard.) From this specification, it is easy to see that the distribution of T at any covariate vector z is exponential with constant hazard (independent of t) λ(t z) =e β 0 β z β pz p. PAGE 03

5 Figure 5.: The density function of the standard extreme value distribution f(x) x So automatically, we get proportional hazards models. For a given set of covariates (z,z 2,..., z p ), the corresponding survival function is S(t z) =e λ(t z)t, where λ(t z) =e β 0 β z β pz p.letβ j = β j.thenequivalently λ(t z) =e β 0 +β z + +β p zp. Therefore, if we increase the value of covariate z k (k =,,p) by on unit from z k to z k + while holding other covariate values fixed, then the ratio of the corresponding hazards is equal to λ(t z k +) λ(t z k ) =e β k. Thus e β k can be interpreted as the hazard ratio corresponding to one unit increase in the covariate z k,orequivalently,β k can be interpreted as the increase in log-hazard as the value of covariate z k increases by one unit (while other covariate values being held constant). Note: Another SAS procedure Proc Phreg fits a proportional hazards model to the data and outputs the regression coefficient estimates in log-hazard form (i.e., in β k). Therefore, if an PAGE 04

6 exponential model fits the data well (Proc Phreg will also fit the data well in this case.) then the regression coefficient estimates in outputs from Proc Lifereg using dist=exponential and Proc Phreg should be just opposite to each other (opposite sign but almost the same absolute value). We should be able to shift back and forth between these two models. Example (Autologous and Allogeneic Bone Marrow Transplants for Hodgkin s and Non- Hodgkin s Lymphoma, pages -2 of the textbook): Data on 43 bone marrow transplant patients were collected. Patients had either Hodgkin s disease or Non-Hodgkin s Lymphoma, and were given either an allogeneic (Allo) transplant (from a HLA match sibling donor) or autogeneic (Auto) transplant (their own marrow was cleansed and returned to them after a high dose of chemotherapy). Other covariates are Karnofsky score (a subjective measure of how well the patient is doing, ranging from 0-00) and waiting time (in months) from diagnosis to transplant. It is of substantial interest to see the difference in leukemia-free survival (in days) between those patients given an Allo or Auto transplant, after adjusting for patients disease status, Karnofsky score and waiting time. The data were given in Table.5 of the textbook. We used the following SAS program to fit an exponential model to the data title "Exponential fit"; proc lifereg data=bone; model time*status(0) = allo hodgkins kscore wtime / dist=exponential; run; where allo= for allogeneic transplant and allo=0 for autologous transplant, hodgkins= for Hodgkin s disease and hodgkins=0 for Non-Hodgkin s Lymphoma, kscore is the Karnofsky score and wtime is the waiting time. We got the following output: Exponential fit 7:35 Tuesday, March, 2005 The LIFEREG Procedure Model Information Data Set WORK.BONE Dependent Variable Log(time) Censoring Variable status Censoring Value(s) 0 PAGE 05

7 Number of Observations Noncensored Values Right Censored Values Left Censored Values 7 0 Interval Censored Values 0 Name of Distribution Exponential Log Likelihood Algorithm converged. Type III Analysis of Effects Wald Effect DF Chi-Square Pr > ChiSq allo hodgkins kscore wtime < Analysis of Parameter Estimates Standard 95% Confidence Chi- Parameter DF Estimate Error Limits Square Pr > ChiSq Intercept allo hodgkins kscore <.000 wtime Scale Weibull Shape Lagrange Multiplier Statistics Parameter Chi-Square Pr > ChiSq Scale According to this model, allogeneic transplant is slightly better than autologous transplant after adjusting for disease status, Karnofsky score and waiting time. Hodgkins patients did worse than Non-Hodgkins patients (the average disease-free survival time for Hodgkins patients is only exp(.385) = 0.27 of that of the Non-Hodgkins patients). The patients with higher Karnofsky scores have better survival (with one point higher of Karnofsky score, the patients average survival time will increase by about 7%). Waiting time has no effect on the disease-free survival. PAGE 06

8 The Weibull model The only difference between the Weibull model and the exponential model is that the scale parameter σ is estimated rather than being set to be one. In this case, the distribution of σε is an extreme value distribution with scale parameter σ. The survival function of T at covariate value z =(,z,,z p ) T can be shown to be { S(t z) =exp [ te ] } zt β σ, where β =(β 0,,β p ) T hazard function is the vector of regression coefficients. Equivalently, in terms of (log)- logλ(t z) = ( σ ) logt logσ z T (β/σ). Let α =/σ, β0 = logσ β 0 /σ, andβj = β j /σ for j =,,p(again, pay close attention to the negative sign). Then we have logλ(t z) =(α )logt + β0 + z β + + z p βp. Thus we also get a proportional hazards model and the coefficient βk (k =,,p)alsohasthe interpretation that it is the increase in log-hazard when the value of covariate z k increases by one unit while other covariate values being held unchanged. The function is the baseline hazard (i.e., whenz =0). λ 0 (t) =t α e β 0 = αt α e β 0/σ = αt α e αβ 0 Note: If the Weibull model is a reasonable model for your data and you use Proc Lifereg and Proc Phreg to fit the data, then the regression coefficient estimates not only have opposite signs (except possibly for the intercept) but also have different magnitude (depending on whether σ>orσ<). Since β k = β k /σ for k =,,p.testingh 0 : β k = 0 is equivalent to testing H 0 : β k =0. If we are interested in calculating standard error for the estimate of β k and constructing a confidence interval for β k, we can use delta method for this purpose. PAGE 07

9 Example (Bone marrow transplant data revisited) We used the following SAS program to fit a Weibull model to the bone marrow transplant data title "Weibull fit"; proc lifereg data=bone; model time*status(0) = allo hodgkins kscore wtime / dist=weibull; run; and got the following output: Weibull fit 2 7:35 Tuesday, March, 2005 The LIFEREG Procedure Model Information Data Set WORK.BONE Dependent Variable Log(time) Censoring Variable status Censoring Value(s) 0 Number of Observations 43 Noncensored Values 26 Right Censored Values 7 Left Censored Values 0 Interval Censored Values 0 Name of Distribution Weibull Log Likelihood Algorithm converged. Type III Analysis of Effects Wald Effect DF Chi-Square Pr > ChiSq allo hodgkins kscore <.000 wtime Analysis of Parameter Estimates Standard 95% Confidence Chi- Parameter DF Estimate Error Limits Square Pr > ChiSq Intercept allo hodgkins kscore <.000 wtime PAGE 08

10 Scale Weibull Shape If we think that the Weibull model is a reasonable one, then the likelihood ratio test statistic is 2*( ( )) = 2.56 and the p-value = 0.096, not a strong evidence against the exponential model. From this model, we see similar results in the transplant methods. The log-normal model The log-normal model simply assumes that ε N(0, ). Let λ 0 (t) be the hazard function of T when β =0(β 0 = β = = β p = 0). Then it can be shown that λ 0 (t) has the following functional form where φ(x) = x2 /2 2π e is λ 0 (t) = φ ( ) log(t) σ [ ( )] Φ log(t), σ σt the probability density function and Φ(x) = x 2π e u2 /2 du is the cumulative distribution function of the standard normal distribution. Then the log-hazard function of T at any covariate value z canbeexpressedas logλ(t z) =logλ 0 (te zt β ) z T β. (5.3) Obviously we no longer have a proportional hazards model. Note: The function λ 0 (t) is not the baseline hazard function. If such a function is desired, it can be obtained from equation (5.3) by setting z =0. Some typical patterns that the hazard function λ 0 (t) assumes are presented in Figure 5.2. The inverted U-shaped of the log-normal hazard if often appropriate for repeated events such as a residential move (i.e, the interest is the time to next move). Immediately after a move, the hazard for another move is likely to be low, then increases with time, and eventually begins to decline since people tend to not move as they get older. The survival function S(t z) at any covariate value z can be expressed as Φ [S(t z)] = β 0 + β z + + β pz p αlog(t), (5.4) PAGE 09

11 Figure 5.2: Typical hazard functions for a log-normal model Hazard functions sigma=0.5 sigma= sigma= time or equivalently S(t z) =Φ[β 0 + β z + + β pz p αlog(t)], where α =/σ and β j = β j /σ for j =0,,,p. Thisisaprobit regression model with intercept depending on t. Note: Equation (5.4) indicates that the coefficients β j can be estimated using Proc Logistic or Proc Genmod by specifying probit link function. Specifically, pick a time point of interest, say t 0. Then dichotomize each subject based on his/her survival status at t 0 (in this case αlog(t 0 ) is absorbed into the intercept). Of cause, there are some limitations using this approach. First, there should not be censoring prior to time t 0. Second, the scale parameter σ is not estimable. Third, we will lose efficiency since we did not use all the information on the exact timing of the events. Since normal distribution and the logistic distribution we will introduce soon behave similarly to each other, the parameters βk here have similar interpretation as the parameters in log-logistic model. Example (Bone marrow transplant data revisited) If we want to fit a log-normal for the bone marrow transplant data, we use the following tt SAS program: title "Log-normal fit"; PAGE 0

12 proc lifereg data=bone; model time*status(0) = run; allo hodgkins kscore wtime / dist=lnormal; The following output is from the above program: Log-normal fit 3 7:35 Tuesday, March, 2005 The LIFEREG Procedure Model Information Data Set WORK.BONE Dependent Variable Log(time) Censoring Variable status Censoring Value(s) 0 Number of Observations 43 Noncensored Values 26 Right Censored Values 7 Left Censored Values 0 Interval Censored Values 0 Name of Distribution Lognormal Log Likelihood Algorithm converged. Type III Analysis of Effects Wald Effect DF Chi-Square Pr > ChiSq allo hodgkins kscore <.000 wtime Analysis of Parameter Estimates Standard 95% Confidence Chi- Parameter DF Estimate Error Limits Square Pr > ChiSq Intercept allo hodgkins kscore <.000 wtime Scale The results from this model are quite similar to the results from other models. PAGE

13 The Log-Logistic Model The log-logistic model assumes that the disturbance term ε has a standard logistic distribution f(ε) = e ɛ ( + e ɛ ) 2. The density is plotted in Figure 5.3. Graphically, it looks like the standard normal density except that the standard normal density puts more mass around its mean value (0). The hazard function of T at any covariate value z has a closed form: where α =/σ. λ(t z) = αtα e zt β/σ +t α e zt β/σ, Figure 5.3: Density function of standard logistic distribution f(x) x Since the logistic distribution looks similar to the normal distribution, it is expected that the hazard function of log-logistic distribution would also look like that of log-normal distribution, i.e., would have an inverted U-shaped hazard. However, this is the case only for σ<. The hazard function of T when β = 0 for some values of σ s is presented in Figure 5.4. PAGE 2

14 Figure 5.4: Hazard functions for the log-logistic distribution Hazard functions sigma=0.5 sigma= sigma= time The random variable T has a very simple survival function at covariate value z S(t z) = +(te zt β ) /σ. Some simple algebra then shows that [ ] S(t z) log = β0 + β S(t z) z + + βpz p αlog(t), (5.5) where β j = β j /σ for j =0,,,p. This is nothing but a logistic regression model with the intercept depending on t. Since S(t z) is the probability of surviving to time t for any given time t, the ratio S(t z)/( S(t z)) is often called the odds of surviving to time t. Therefore, with one unit increase in z k while other covariates being held fixed, the odds ratio is given by S(t z k +)/( S(t z k +)) S(t z k )/( S(t z k )) =e β k for all t 0, which is a constant over time. Therefore, we have a proportional odds models. Hence βk can be interpreted as the log odds ratio (for surviving) with one unit increase in z k and β is the log odds ratio of dying before time t with one unit increase in z k. At the times when the event of failure is rare (such as the early phase of a study), βk can also be approximately interpreted as the log relative risk of dying. The log-logistic model is the only one that is both an AFT model and a proportional odds model. PAGE 3

15 Obviously, ε has the following cumulative distribution function whose inverse function is often called the logit function. F (u) = [ logit(π) =log eu, u (, ), +eu π π ], π (0, ), Note: As in the case of log-normal distribution, equation (5.5) indicates that the regression coefficients β k can be estimated using Proc logistic or Proc Genmod. See the notes for lognormal model for the procedure and the limitations of such approach. Example (Bone marrow transplant data revisited) We fit a log-logistic model to the data using the following SAS program: title "Log-logistic fit"; proc lifereg data=bone; model time*status(0) = allo hodgkins kscore wtime / dist=llogistic; run; and got the following output: Log-logistic fit 4 7:35 Tuesday, March, 2005 The LIFEREG Procedure Model Information Data Set WORK.BONE Dependent Variable Log(time) Censoring Variable status Censoring Value(s) 0 Number of Observations 43 Noncensored Values 26 Right Censored Values 7 Left Censored Values 0 Interval Censored Values 0 Name of Distribution LLogistic Log Likelihood Algorithm converged. PAGE 4

16 Type III Analysis of Effects Wald Effect DF Chi-Square Pr > ChiSq allo hodgkins kscore <.000 wtime Analysis of Parameter Estimates Standard 95% Confidence Chi- Parameter DF Estimate Error Limits Square Pr > ChiSq Intercept allo hodgkins kscore <.000 wtime Scale We got the same conclusion that two transplants are not significantly different after adjusting for other covariates. The effects of other covariates are similar too. The gamma model The procedure Proc Lifereg in SAS actually fits a generalized gamma model (not a standard gamma model) to the data by assuming T 0 =e ε to have the following density function f(t) = δ (t δ /δ 2 ) /δ2 exp( t δ /δ 2 )/(tγ(/δ 2 )), where δ is called shape with label Gamma shape parameter in the output of Proc Lifereg when we specify dist=gamma. The hazard function of this gamma distribution does not have a closed form and is presented graphically in Figure 5.5 for some δ s. Note: Theyarenot the hazard functions of the standard gamma distribution. Clearly from this plot, the hazard function is an inverted U-shaped function of time when δ< and takes a U-shape when δ>. This feature makes gamma model very appropriate to model the survival times, especially for human. In practice, the hazard function is determined jointly by the scale parameter σ and the shape parameter δ and we need to examine the resulting PAGE 5

17 Figure 5.5: Hazard functions for the gamma distribution Hazard functions delta=0.5 delta=.5 delta= time hazard function case by case. For a given set of covariates (z,z 2,..., z p ), let c = e β 0+z β +...+z k β k = e z T β. Then log(t )= z T β + σɛ implies T = e zt β [e ɛ ] σ = ct σ 0. Hence the survival function for this population is S(t z) = P [T t] =P [ct σ 0 t] = P [T 0 (c t) /σ ]=P [T 0 b(t)] = x δ /δ 2 =y = b(t) δ (x δ /δ 2 ) /δ2 exp( x δ /δ 2 )/(xγ(/δ 2 ))dx αb(t) δ yα e y Γ(α) dy if δ>0 αb(t) δ 0 y α e y Γ(α) dy if δ<0, where α =/δ 2. This final integration can be calculated using built-in functions in any popular software. Example (Bone marrow transplant data revisited) We fit a gamma model to the bone marrow transplant data using the following SAS program title "Gamma fit"; proc lifereg data=bone; model time*status(0) = allo hodgkins kscore wtime / dist=gamma; run; and got the following results: PAGE 6

18 Gamma fit 5 7:35 Tuesday, March, 2005 The LIFEREG Procedure Model Information Data Set WORK.BONE Dependent Variable Log(time) Censoring Variable status Censoring Value(s) 0 Number of Observations 43 Noncensored Values 26 Right Censored Values 7 Left Censored Values 0 Interval Censored Values 0 Name of Distribution Gamma Log Likelihood Algorithm converged. Type III Analysis of Effects Wald Effect DF Chi-Square Pr > ChiSq allo hodgkins kscore wtime Analysis of Parameter Estimates Standard 95% Confidence Chi- Parameter DF Estimate Error Limits Square Pr > ChiSq Intercept allo hodgkins kscore wtime Scale Shape Again, there is no significant different in the transplant methods. Some special cases:. δ = T z has the Weibull distribution. 2. δ = 0 T z has the log-normal distribution. We need the following approximation in PAGE 7

19 order to show this: Γ(x) 2πx x 2 e x as x. 3. δ = σ T z has the standard gamma distribution with the following density: f(t z) = tk e t γ γ K Γ(K), where K =/δ 2 is the shape parameter and γ = δ 2 e zt β is the scale parameter in the standard gamma distribution. However, Proc Lifereg will not fit a standard gamma distribution to data. We have to use grid search to fit a standard gamma distribution. Specifically, use the output from a generalized gamma distribution to get an idea about the true value of σ = δ. Then form a grid. For each grid point of σ (or δ), say, σ =.2, fit a gamma distribution with the specification dist=gamma noshape shape=.2 noscale scale=.2;. Select the model that gives the largest log-likelihood. Categorical Variables and Class statement in Proc Lifereg Ifacovariatez k is categorical, then we can use class statement in Proc Lifereg to tell the procedure that z k is categorical and just enter z k in the model statement in a usual way. Then the output of Proc Lifereg will provide a χ 2 statistic and a p-value to test the null hypothesis that the survival time is not associated with z k. Then it reports the estimates, standard errors, and p-values, etc., for the contrasts between each level and the highest level of the category. However, we need to create appropriate variables for the interaction. 5.3 Goodness-of-fit Using Likelihood Ratio Test The following fact on the models we described in this chapter allows us to perform likelihood ratio tests in order to pick a right model:. generalized gamma (δ, σ) standard gamma (δ = σ) exponential distribution (δ = σ = ). PAGE 8

20 2. generalized gamma (δ, σ) Weibull (δ =,σ) exponential distribution (δ = σ =). 3. generalized gamma (δ, σ) log-normal (δ =0,σ). We can use the above nested models to conduct likelihood ratio test for the bone marrow transplant data: Maximum likelihood values for different models Model Maximum Likelihood Gamma Log-logistic -6.5 Log-normal Weibull -6.2 Exponential Assuming the Gamma model is a reasonable model for the data, the LRT indicates that the log-normal model is equally good and the Weibull model is also acceptable. Since the log-normal model and the Weibull model have the same number of parameters, we might want to take the log-normal model as the final model based on the larger maximum log-likelihood value. PAGE 9

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