# Univariate shrinkage in the Cox model for high dimensional data

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1 Univariate shrinkage in the Cox model for high dimensional data Robert Tibshirani January 6, 2009 Abstract We propose a method for prediction in Cox s proportional model, when the number of features (regressors) p exceeds the number of observations n. The method assumes that features are independent in each risk set, so that the partial likelihood factors into a product. As such, it is analogous to univariate thresholding in linear regression and nearest shrunken centroids in classification. We call the procedure Cox univariate shrinkage and demonstrate its usefulness on real and simulated data. The method has the attractive property of being essentially univariate in its operation: the features are entered into the model based on the size of their Cox score statistics. We illustrate the new method on real and simulated data, and compare it to other proposed methods for survival prediction with a large number of predictors. Keywords: survival data, Proportional hazards model, Cox model, shrinkage, subset selection 1 Introduction High-dimensional regression problems are challenging, and a current focus of statistical research. One simplifying assumption that can be useful is that of independence of features. While this assumption will rarely be even approximately true, in practice, it leads to simple estimates with predictive performance sometimes as good or better than more ambitious multivariate methods. Depts. of Health, Research & Policy, and Statistics, Stanford Univ, Stanford CA, 94305; 1

2 In linear regression with squared error loss and a lasso penalty, independence of the features leads to the univariate soft-thresholding estimates, that often behave well. In classification problems, the idea of class-wise independence forms the basis for diagonal discriminant analysis. In that setting, if we assume that the features are independent within each class, and incorporate a lasso penalty, the resulting estimator is the nearest shrunken centroid procedure proposed by Tibshirani et al. (2001): we review this fact in the next section. The nearest shrunken centroid classifier is very simple but useful in practice, and has many attractive properties. It has a single tuning parameter that automatically selects the features to retain in the model, and it provides a single ranking of all features, that is independent of the value of the tuning parameter. We review the details of the regression and classification problems in the next section. Then we extend the idea of class-wise feature independence to the Cox model for survival data: this is the main contribution of the paper. We study this new procedure on both real and simulated datasets, and compare it to competing methods for this problem. 2 Regression and classification using feature independence 2.1 Regression Consider the usual regression situation: we have data (x i, y i ), i = 1, 2,... n where x i = (x i1,... x ip ) T and y i are the regressors and response for the ith observation. Assume the x ij are standardized so that i x ij/n = 0, i x2 ij /n = 1. The lasso finds β = (β 1,... β p ) T to minimize N i=1 (y i j p β j x ij ) 2 + λ β j. (1) j=1 Now if we assume that the features are independent (uncorrelated), it is easy to show that ˆβ j = sign( ˆβ o j )( ˆβ o j λ) + (2) where ˆβ o j are the ordinary (univariate) least squares estimates. These are univariate soft-thresholded estimates, and perform particularly well when p n. Zou & Hastie (2005) study these estimates as a special case of their elastic net procedure. See Fan & Fan (2008) for some theoretical support of univariate soft-thresholding. 2

3 2.2 Classification Here we review the nearest shrunken centroid method for high-dimensional classification and its connection to the lasso. Consider the diagonal-covariance linear discriminant rule for classification. The discriminant score for class k is p (x δ k (x j x kj ) 2 ) = + 2 log π k. (3) j=1 Here x = (x 1, x 2,..., x p )T is a vector of expression values for a test observation, s j is the pooled within-class standard deviation of the jth gene, and x kj = i C k x ij /n k is the mean of the n k values for gene j in class k, with C k being the index set for class k. We call x k = ( x k1, x k2,... x kp ) T the centroid of class k. The first part of (3) is simply the (negative) standardized squared distance of x to the kth centroid. The second part is a correction based on the class prior probability π k, where K k=1 π k = 1. The classification rule is then s 2 j C(x ) = l if δ l (x ) = max k δ k (x ). (4) We see that the diagonal LDA classifier is equivalent to a nearest centroid classifier after appropriate standardization. The nearest shrunken centroid classifier regularizes this further, and is defined as follows. Let d kj = x kj x j m k (s j + s 0 ), (5) where x j is the overall mean for gene j, m 2 k = 1/n k 1/n and s 0 is a small positive constant, typically chosen to be the median of the s j values. This constant guards against large d kj values that arise from expression values near zero. Finally, we can derive the nearest shrunken centroid classifier as the solution to a lasso-regularized problem that assumes independence of the features. Consider a (naive Bayes) Gaussian model for classification in which the features j = 1, 2,..., p are assumed to be independent within each class k = 1, 2,..., K. With observations i = 1, 2,..., n and C k equal to the set of indices of the n k observations in class k, we observe x ij N(µ j + µ jk, σj 2) for i C k with K k=1 µ jk = 0. We set ˆσ j 2 = s2 j, the pooled within-class variance for feature j, and consider the lasso-style minimization problem { 1 p K min {µ j,µ jk } 2 j=1 k=1 (x ij µ j µ jk)2 s 2 i C j k + λ n k p K j=1 k=1 } µ jk. (6) s j By direct calculation, it is easy to show that the solution is equivalent to the nearest shrunken centroid estimator (5), with s 0 set to zero, and M k equal to 1/n k instead of 1/n k 1/n as before. Some interesting theoretical results for diagonal linear discriminant analysis are given in Bickel & Levina (2004). Efron (2008) proposes an Empirical Bayes variation on nearest shrunken centroids. 3

4 3 Survival data and Cox s proportional hazards model For the rest of this paper we consider the right-censored survival data setting. The data available are of the form (y 1, x 1, δ 1, ),..., (y n, x n, δ n ), the survival time y i being complete if δ i = 1 and right censored if δ i = 0, with x i denoting the usual vector of predictors (x 1, x 2,... x p ) for the ith individual. Denote the distinct failure times by t 1 < < t K, there being d k failures at time t k. The proportional-hazards model for survival data, also known as the Cox model, assumes that ( ) λ(t x) = λ 0 (t) exp x j β j (7) where λ(t x) is the hazard at time t given predictor values x = (x 1,..., x p ), and λ 0 (t) is an arbitrary baseline hazard function. One usually estimates the parameter β = (β 1, β 2,... β p ) T in the proportionalhazards model (7) without specification of λ 0 (t) through maximization of the partial likelihood PL(β) = exp(x k β) { k D m R k exp(x m β)}. Here D is the set of indices of the failure times, R k is the set of indices of the individuals at risk at time t k 0, and we have assumed there are no ties in the survival times. In the case of ties, we use the Breslow approximation to the partial likelihood. We also assume that the censoring is noninformative, so that the construction of the partial likelihood is justified. If p n then maximizer of the partial likelihood is not unique, and some regularization must be used. One possibility is to include a lasso penalty and jointly optimize over the parameters, as suggested by Tibshirani (1997). Here we extend the idea of feature independence to the Cox model for survival data. Now the partial likelihood term exp(x k β)/ m R k exp(x m β) equals Pr(k x i, i R k ) the probability that individual k is the one who fails at time t k, given the risk set and their feature vectors. Now we assume that both conditionally on each risk set, and marginally, the features are independent of one another. Then using Bayes theorem we obtain Pr(k x i, i R k ) j Pr(k x ij, i R k ). As a result, up to a proportionality constant, the partial likelihood becomes a simple product p exp(x kj β j ) PL(β) = c { m R k exp(x mj β j )} j=1 k D j 4

5 The log partial likelihood is l(β) = p K xkj β j log k=1( m R k p g j (β j ). (9) j=1 j=1 exp(x mj β j ) ) (8) We propose the Cox univariate shrinkage () estimator as the maximizer of the penalized partial log-likelihood J(β) = p g j (β j ) λ β j. (10) j=1 Here λ 0 is a tuning parameter, and we solve this problem for a range of λ values. Note that is just a set of one-dimensional maximizations, since we can maximize each function g j (β j ) λ β j separately. The minimizers of (10) are not simply soft-thresholded versions of the unpenalized estimates ˆβ j, as they are in the least squares regression case. We discuss and illustrate this fact in the next section. From this, we obtain solutions paths ( ˆβ 1 (λ), ˆβ 2 (λ),... ˆβ p (λ)), and we can use these to predict the survival in a separate test sample. Note that the estimates ˆβ will tend to be biased towards zero, especially for larger values of λ. In the test set and cross-validation computations in this paper, we fit the predictor z = x ˆβ in the new (test or validation) data, and so obtain a scale factor to debias the estimate. If we are interested in the values of ˆβ themselves, a better approach would be fit the single predictor γx i ˆβ in a Cox model on the training data, and hence obtain the adjusted estimates ˆγ ˆβ. The lasso penalty above might not make sense if the predictors are in different units. Therefore we first standardize each predictor x j by s j, the square root of the observed (negative) Fisher information at β j = Example Zhao et al. (2005) collected gene expression data on 14, 814 genes from 177 kidney patients. Survival times (possibly censored) were also measured for each patient. The data were split into 88 samples to form the training set and the remaining 89 formed the test set. Figure 1 shows the solution paths for Cox univariate shrinkage for 500 randomly chosen genes. Notice that the solutions paths have a particularly nice form: they are monotonically decreasing in absolute value and once they hit zero, they stay at zero. It is easy to prove that this holds in general. The proof is given in the Appendix. Note also that the profiles decrease at different rates. This is somewhat surprising. In the least squares regression case under orthogonality of the predictors, 5

6 Coefficients PSfrag replacements λ Figure 1: Kidney cancer example: solution paths for univariate shrinkage, for 500 randomly chosen genes. the lasso estimates are simply the soft-thresholded versions of the least squares estimates ˆβ j :, that is, sign( ˆβ j )( ˆβ j t) +. Thus every profile decreases at the same rate. Here, each profile decreases at rate 1/ g ( ˆβ j ), which is the reciprocal of the observed Fisher information. Let U j, V j be the gradient of the (unpenalized) log-partial likelihood and the (negative) observed Fisher information. Then we can also show that ˆβ λ 0 U j / V j > λ. (11) The proof is given in the Appendix. Therefore we have the following useful fact: For any value of λ, the set of predictors that appear in the model estimated by Cox univariate shrinkage are just those whose score statistic exceeds λ in absolute value. That is, the Cox univariate shrinkage method used a fixed ranking of all features based on the Cox score statistic. This is very convenient for interpretation, as the Cox score is often used for determining the univariate significance of features (e.g. in the SAM procedure of Tusher et al. (2001)). However the non-zero coefficients given to these predictors are not simply the score statistics or soft thresholded versions of them. Figure 3 shows the drop in test sample deviance for (Cox univariate shrinkage), lasso, SPC (Supervised principal components) and UST (univariate 6

7 Drop in deviance Number of non zero genes Figure 2: Kidney cancer example: cross-validation curve. Drop in deviance on validation set, with plus and minus one standard error bars. soft thresholding), plotted against the number of genes in the model. The and SPC methods work best. Figure 4 shows the univariate Cox coefficients plotted against the Cox score statistics, for the kidney cancer data. Although the two measures are correlated, this correlation is far from perfect, and again this underlines the difference between our proposed univariate shrinkage method, which enters features based on the size of the Cox score, and simple soft-thresholding of the univariate coefficients. The latter ranks features based on the size of their unregularized coefficient, while the former looks at the standardized effect size at the null end of the scale. To estimate the tuning parameter λ we can use cross-validation. However it is not clear how to apply cross-validation in the setting of partial likelihood, since this function is not a simple sum of independent terms. Verweij & van Houwelingen (1993) propose an interesting method for leave-one-out cross-validation of the partial likelihood: for each left-out observation they compute the probability of its sample path over the risk sets. For the fitting methods presented in this paper, this leave-one-out cross-validation was too slow, requiring too many model fits. When we instead tried 5 or 10- fold partial likelihood cross-validation, the resulting curve was quite unstable. So we settled on a simpler approach, 5-fold cross-validation where we fit a Cox model to the left out one-fifth and record the drop in deviance. The results for the kidney cancer data is shown in Figure 2. The curve is a reasonable estimate of the test deviance curve of Figure 3. In some microarray datasets, there are duplicate genes, or multiple clones representing the same gene. In this case, the data for the different features will 7

8 Drop in test set deviance SPC UST Figure 3: Kidney cancer example: drop in test sample deviance for (Cox univariate shrinkage), lasso, SPC (Supervised principal components) and UST (univariate soft thresholding). Univariate Cox coefficients Cox score Figure 4: Kidney cancer data: the univariate Cox coefficients plotted against the Cox score statistics. 8

9 be highly correlated, and it makes sense to average the data for each gene, before applying UC. One can also average genes whose pairwise correlation is very high, whether or not they re supposed to represent the same gene. For example, when we averaged genes with correlation greater than 0.9 in the kidney cancer dataset, it reduced the number of genes by a few hundred and caused very little change in the test deviance curve. 3.2 Selection of a more parsimonius model The procedure enters predictors into the model based on this univariate Cox scores. Thus if two predictors are strongly predictive and correlated with each other, both will appear in the model. In some situations it is desirable pare down the predictors to a smaller, mode independent set. The lasso does this selection automatically, but sometimes doesn t predict as well as other methods (as in Figure 3.) An alternative approach is pre-conditioning (Paul et al. 2008), in which we apply the standard (regression) lasso to the fitted values from a model. In Paul et al. (2008) the initial fitting method used was supervised principal components. Here we start with the fit using 500 genes, which approximately the best model from Figure 2. We then apply the (regression version) of the lasso to the fitted values, and we report the drop in test deviance in Figure 5, along with that for itself (green curve copied from Figure 3.) We see that pre-conditioning gives about the same drop in test deviance as the 500 gene model, but using fewer than 100 genes. And it performs better here than the Cox model lasso (blue curve in Figure 3, a finding supported theoretically in Paul et al. (2008). Figure 6 shows the univariate Cox scores for the predictors entered by each method. We see that the pre-conditioning approach enters only predictors that are individually strong, while the usual lasso enter many predictors that are individually weak. 4 Simulation study We generated Gaussian samples of size n = 50 with p = 1000 predictors, with correlation ρ(x i, x j ) between each pair of predictors. The outcome y was generated as an exponential random variable with mean exp( x j β j ), with a randomly chosen s out of the p coefficients equal to value ±β and the rest equal to zero. We considered two scenarios s = 20 and s = 200, and tried both ρ(x i, x j ) = 0 and ρ(x i, x j ) = 0.5 i j. Three fitting methods for the Cox model were compared: lasso, supervised principal components, and, over a range of the tuning parameter for each method. A test set of size 400 was generated and the test set deviance of each fitting model was computed. Figure 7 shows the median test deviance plus and minus one standard error over 10 simulations, plotted against the number of genes in each model. 9

10 Drop in test set deviance Pre cond Figure 5: Kidney cancer example: drop in test sample deviance for (Cox univariate shrinkage), and pre-conditioning applied to. In the latter we apply the regression version of the lasso to the fitted values from. Cox model lasso applied to fit Univariate Cox score Univariate Cox score Number of features entered Number of features entered Figure 6: Kidney cancer example: univariate Cox scores for all predictors (black) and those entered into the model (red). Panel on the left corresponds to L 1 - penalized partial likelihood, fit by coxpath. On the right is the standard regression version of the lasso applied to the fitted values from. 10

11 Corr= 0, #nonzero= 20 Corr= 0.5, #nonzero= 20 Drop in test set deviance SPC Drop in test set deviance SPC Corr= 0, #nonzero= 200 Corr= 0.5, #nonzero= 200 Drop in test set deviance SPC Drop in test set deviance SPC Figure 7: Simulation results: average drop in test set deviance (± one standard error), for three different estimators, over four different simulation settings. 11

12 Figure 8 shows the number of genes correctly included in the model versus the number of genes in the model, for each of and lasso. Neither method is very good at isolating the true non-zero predictors, but surprisingly the lasso does no better than the simple procedure. Corr= 0, #nonzero= 20 Corr= 0.5, #nonzero= 20 Number correct Number correct chosen chosen Corr= 0, #nonzero= 200 Corr= 0.5, #nonzero= 200 Number correct Number correct chosen chosen Figure 8: Simulation results: number of correctly included predictors in the model, for and lasso, over the four different simulation settings. 12

13 5 Some real data examples We applied three methods- lasso, supervised principal components, and, to three real datasets. The first is the kidney data discussed earlier; the other datasets are the diffuse large cell lymphoma data of Rosenwald et al. (2002) (7399 genes and 240 patients), and the breast cancer data of van t Veer et al. (2002) (24881 genes, 295 patients). We randomly divided each dataset into training and test sets ten times, and averaged the results. The first two datasets included test sets of size 88 and 80 respectively, so we retained those test set sizes. For the third dataset, we took a test set of about one-third (98 patients), and to ease the computation load, restricted attention to the 10, 000 genes with largest variance. Figure 9 shows the median test deviance plus and minus one standard error over 10 simulations, plotted against the number of genes in each model. The method yields a larger drop in test deviance than the lasso, in at least two of the datasets and performs a little better than SPC overall. Acknowledgments. I thank Daniela Witten for helpful comments, and acknowledge support from National Science Foundation Grant DMS and National Institutes of Health Contract N01-HV Appendix Monotonicity of profiles. The subgradient equation for each β j is: K ( m R s kj d k x mj exp(x mj β j )) k λ t j = 0 (12) m R k exp(x mj β j ) k=1 where t j sign(β j ), that is t j = sign(β j ) if β j 0 and t j [ 1, 1] if β j = 0 Claim: ˆβ j (λ) is strictly decreasing in λ when ˆβ j (λ) is non-zero, and if ˆβ j (λ) = 0 then ˆβ j (λ ) = 0 for all λ > λ. Proof: for ease of notation, suppose we have a single β having the sub-gradient equation g(β) λt = 0 (13) with t sign(β). Then g (β) is the negative of the observed information and it is easy to show that g (β) < 0. Denote the solution to (13) for parameter λ by ˆβ(λ). Suppose that we have a solution ˆβ(λ) 0 for some λ. WLOG assume ˆβ(λ) > 0. Then by (13) we have g( ˆβ(λ)) = λ 13

14 Drop in test set deviance SPC Drop in test set deviance SPC Drop in test set deviance SPC Figure 9: Top to bottom: results for Kidney, Lymphoma and Breast cancer datasets; shown is the average drop in test set deviance (± one standard error) for (Cox univariate shrinkage), lasso and SPC (supervised principal components). 14

15 Then if λ > λ, we can t have ˆβ(λ ) ˆβ(λ) since this would imply g( ˆβ(λ )) < g( ˆβ(λ)) = λ < λ. Hence ˆβ(λ ) < ˆβ(λ). On the other hand, if ˆβ(λ) = 0 then g(0) λt = 0 for some t with t 1. Then if λ > λ the equation g(0) λ t = 0 can be solved by taking t = t(λ/λ ). Hence ˆβ(λ ) = 0. Proof of (11) Let f j (β j ) = K ( m R s kj d k x mj exp(x mj β j )) k m R k exp(x mj β j ) k=1 Suppose ˆβ j (0) > 0. Then ˆβ j (λ) 0 the equation f j (β j ) λ = 0 has a solution in (0, ˆβ j (0)). Since the x j have been standardized by V j, this equivalent to f j (0)/ V j > λ. This latter quantity is the (square root) of the usual score statistic for β j = 0. References Bickel, P. & Levina, E. (2004), Some theory for Fisher s linear discriminant function, naive Bayes, and some alternatives when there are many more variables than observations, Bernoulli 10, Efron, B. (2008), Empirical bayes estimates for large scale prediction problems. Unpublished. Fan, J. & Fan, Y. (2008), Sure independence screening for ultra-high dimensional feature space, Journal of the Royal Statistical Society Series B, to appear. Paul, D., Bair, E., Hastie, T. & Tibshirani, R. (2008), pre-conditioning for feature selection and regression in high-dimensional problems, Annals of Statistics 36(4), Rosenwald, A., Wright, G., Chan, W. C., Connors, J. M., Campo, E., Fisher, R. I., Gascoyne, R. D., Muller-Hermelink, H. K., Smeland, E. B. & Staudt, L. M. (2002), The use of molecular profiling to predict survival after chemotherapy for diffuse large b-cell lymphoma, The New England Journal of Medicine 346, Tibshirani, R. (1997), The lasso method for variable selection in the cox model, Statistics in Medicine 16,

16 Tibshirani, R., Hastie, T., Narasimhan, B. & Chu, G. (2001), Diagnosis of multiple cancer types by shrunken centroids of gene expression, Proceedings of the National Academy of Sciences 99, Tusher, V., Tibshirani, R. & Chu, G. (2001), Significance analysis of microarrays applied to transcriptional responses to ionizing radiation, Proc. Natl. Acad. Sci. USA. 98, van t Veer, L. J., van de Vijver, H. D. M. J., He, Y. D., Hart, A. A., Mao, M., Peterse, H. L., van der Kooy, K., Marton, M. J., Anke T. Witteveen and, G. J. S., Kerkhoven, R. M., Roberts, C., Linsley, P. S., Bernards, R., & Friend, S. H. (2002), Gene expression profiling predicts clinical outcome of breast cancer, Nature 415, Verweij, P. & van Houwelingen, H. (1993), Cross-validation in survival analysis, Statistics in Medicine 12, Zhao, H., Tibshirani, R. & Brooks, J. (2005), Gene expression profiling predicts survival in conventional renal cell carcinoma, PLOS Medicine pp Zou, H. & Hastie, T. (2005), Regularization and variable selection via the elastic net, Journal of the Royal Statistical Society Series B. 67(2),

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