Survival Distributions (survival + distribution)

Distribution by Scientific Domains


Selected Abstracts


Five-Year Survival Distributions of Short-Length (10 mm or less) Machined-Surfaced and Osseotite® Implants

CLINICAL IMPLANT DENTISTRY AND RELATED RESEARCH, Issue 1 2004
Sylvan Feldman DDS
ABSTRACT Background: In cases of reduced alveolar bone height, implants of short length (10 mm or less) may be employed although there is a perceived risk that because of their small stature they will be unable to tolerate occlusal loads and will fail to osseointegrate. Purpose: This report describes an analysis of prospective multicenter clinical studies evaluating the risk for failure of short-length implants, comparing dual acid-etched (DAE) Osseotite® implants (Implant Innovations, Inc., Palm Beach Gardens, FL, USA) to machined-surfaced implants. Materials and Methods: Admission criteria were the same for both data sets. Baseline variables of demographics including age, gender and smoking status, bone quality, location, implant dimensions, and types of prostheses were compared to ensure balance among groups. Cumulative survival rates (CSRs) were calculated with the Kaplan-Meier estimator. Results: The implant data included 2,294 implants for the DAE series and 2,597 implants for the machined-surfaced series. Patient demographics showed similar percentages of occurrence for all variables. The distributions of implants between short- and standard-length data sets for baseline variables including width, location, and restorative type were similar, qualifying these data sets for comparison of the independent variable of length. Overall, there was a 2.2% difference in 5-year CSRs between the machined-surfaced short- and the standard-length implants. For these implants a 7.1% difference was observed in the posterior maxilla and an 8.5% difference in the anterior maxilla. For DAE implants the overall difference between "standards" and "shorts" was 0.7%, which is not statistically significant. Conclusion: In this analysis the difference in CSRs between short- and standard-length implants was greater for machined-surfaced implants than for DAE implants. [source]


Recall event timing: Measures of managerial performance in U.S. meat and poultry plants

AGRIBUSINESS : AN INTERNATIONAL JOURNAL, Issue 3 2005
Ratapol Teratanavat
This study investigates the performance of meat and poultry plant managers in discovering and responding effectively to food safety problems that lead to product recalls. Timing is used as a performance measure of managers' response to recalls of food, using survival distributions of times between production and recall, and recall case duration. The objectives are to understand how these time periods vary across plants and to determine factors explaining such variability. Survival distributions are estimated using the Kaplan-Meier and life table methods. Subgroups of the population are compared using plots of the estimated survival functions and statistically compared using log-rank and Wilcoxon tests. Managers at large plants, in multi-plant firms, and at plants with prior recall experience do not perform better. Cox regressions indicate that government agency sampling programs enhanced the speed of discovery, and that national distribution networks contributed to the risk that cases remained open for a longer period. [EconLit citations: D210, Q180.] © 2005 Wiley Periodicals, Inc. Agribusiness 21: 351,373, 2005. [source]


Flexible Estimation of Differences in Treatment-Specific Recurrent Event Means in the Presence of a Terminating Event

BIOMETRICS, Issue 3 2009
Qing Pan
Summary In this article, we consider the setting where the event of interest can occur repeatedly for the same subject (i.e., a recurrent event; e.g., hospitalization) and may be stopped permanently by a terminating event (e.g., death). Among the different ways to model recurrent/terminal event data, the marginal mean (i.e., averaging over the survival distribution) is of primary interest from a public health or health economics perspective. Often, the difference between treatment-specific recurrent event means will not be constant over time, particularly when treatment-specific differences in survival exist. In such cases, it makes more sense to quantify treatment effect based on the cumulative difference in the recurrent event means, as opposed to the instantaneous difference in the rates. We propose a method that compares treatments by separately estimating the survival probabilities and recurrent event rates given survival, then integrating to get the mean number of events. The proposed method combines an additive model for the conditional recurrent event rate and a proportional hazards model for the terminating event hazard. The treatment effects on survival and on recurrent event rate among survivors are estimated in constructing our measure and explain the mechanism generating the difference under study. The example that motivates this research is the repeated occurrence of hospitalization among kidney transplant recipients, where the effect of expanded criteria donor (ECD) compared to non-ECD kidney transplantation on the mean number of hospitalizations is of interest. [source]


Legendre polynomial kernel estimation of a density function with censored observations and an application to clinical trials

COMMUNICATIONS ON PURE & APPLIED MATHEMATICS, Issue 8 2007
Simeon M. Berman
Let f(x), x , ,M, M , 1, be a density function on ,M, and X1, ,., Xn a sample of independent random vectors with this common density. For a rectangle B in ,M, suppose that the X's are censored outside B, that is, the value Xk is observed only if Xk , B. The restriction of f(x) to x , B is clearly estimable by established methods on the basis of the censored observations. The purpose of this paper is to show how to extrapolate a particular estimator, based on the censored sample, from the rectangle B to a specified rectangle C containing B. The results are stated explicitly for M = 1, 2, and are directly extendible to M , 3. For M = 2, the extrapolation from the rectangle B to the rectangle C is extended to the case where B and C are triangles. This is done by means of an elementary mapping of the positive quarter-plane onto the strip {(u, v): 0 , u , 1, v > 0}. This particular extrapolation is applied to the estimation of the survival distribution based on censored observations in clinical trials. It represents a generalization of a method proposed in 2001 by the author [2]. The extrapolator has the following form: For m , 1 and n , 1, let Km, n(x) be the classical kernel estimator of f(x), x , B, based on the orthonormal Legendre polynomial kernel of degree m and a sample of n observed vectors censored outside B. The main result, stated in the cases M = 1, 2, is an explicit bound for E|Km, n(x) , f(x)| for x , C, which represents the expected absolute error of extrapolation to C. It is shown that the extrapolator is a consistent estimator of f(x), x , C, if f is sufficiently smooth and if m and n both tend to , in a way that n increases sufficiently rapidly relative to m. © 2006 Wiley Periodicals, Inc. [source]


MNS16A minisatellite genotypes in relation to risk of glioma and meningioma and to glioblastoma outcome

INTERNATIONAL JOURNAL OF CANCER, Issue 4 2009
Ulrika Andersson
Abstract The human telomerase reverse transcriptase (hTERT) gene is upregulated in a majority of malignant tumours. A variable tandem repeat, MNS16A, has been reported to be of functional significance for hTERT expression. Published data on the clinical relevance of MNS16A variants in brain tumours have been contradictory. The present population-based study in the Nordic countries and the United Kingdom evaluated brain-tumour risk and survival in relation to MNS16A minisatellite variants in 648 glioma cases, 473 meningioma cases and 1,359 age, sex and geographically matched controls. By PCR-based genotyping all study subjects with fragments of 240 or 271 bp were judged as having short (S) alleles and subjects with 299 or 331 bp fragments as having long (L) alleles. Relative risk of glioma or meningioma was estimated with logistic regression adjusting for age, sex and country. Overall survival was analysed using Kaplan,Meier estimates and equality of survival distributions using the log-rank test and Cox proportional hazard ratios. The MNS16A genotype was not associated with risk of occurrence of glioma, glioblastoma (GBM) or meningioma. For GBM there were median survivals of 15.3, 11.0 and 10.7 months for the LL, LS and SS genotypes, respectively; the hazard ratio for having the LS genotype compared with the LL was significantly increased HR 2.44 (1.56,3.82) and having the SS genotype versus the LL was nonsignificantly increased HR 1.46 (0.81,2.61). When comparing the LL versus having one of the potentially functional variants LS and SS, the HR was 2.10 (1.41,3.1). However, functionality was not supported as there was no trend towards increasing HR with number of S alleles. Collected data from our and previous studies regarding both risk and survival for the MNS16A genotypes are contradictory and warrant further investigations. © 2009 UICC [source]


Recall event timing: Measures of managerial performance in U.S. meat and poultry plants

AGRIBUSINESS : AN INTERNATIONAL JOURNAL, Issue 3 2005
Ratapol Teratanavat
This study investigates the performance of meat and poultry plant managers in discovering and responding effectively to food safety problems that lead to product recalls. Timing is used as a performance measure of managers' response to recalls of food, using survival distributions of times between production and recall, and recall case duration. The objectives are to understand how these time periods vary across plants and to determine factors explaining such variability. Survival distributions are estimated using the Kaplan-Meier and life table methods. Subgroups of the population are compared using plots of the estimated survival functions and statistically compared using log-rank and Wilcoxon tests. Managers at large plants, in multi-plant firms, and at plants with prior recall experience do not perform better. Cox regressions indicate that government agency sampling programs enhanced the speed of discovery, and that national distribution networks contributed to the risk that cases remained open for a longer period. [EconLit citations: D210, Q180.] © 2005 Wiley Periodicals, Inc. Agribusiness 21: 351,373, 2005. [source]


Craniofacial Resection for Nonmelanoma Skin Cancer of the Head and Neck,

THE LARYNGOSCOPE, Issue 6 2005
Douglas D. Backous MD
Abstract Objectives/Hypothesis: We reviewed our experience with craniofacial resection for advanced, nonmelanoma skin cancer of the head and neck to determine prognostic factors, local control rate, disease free survival, morbidity, and mortality. Study Design: Retrospective review of consecutive patients treated at a tertiary referral center from 1982 to 1993. Methods: Charts of patients having craniofacial resection for aggressive nonmelanoma cutaneous malignancies were reviewed and living patients followed for 10 additional years. Demographics, histology, previous interventions, treatment, surgical pathology, reconstructions, and complications were examined. The product-limit method was used to calculate survival functions, and the log-rank test was used to compare survival distributions. Results: Thirty-five patients, mean age 66.7 years, received treatment at our facility. Follow-up ranged from 2 to 191 (mean 47.4) months. Histology included 20 squamous cell carcinomas (SCC) and 15 basal cell carcinomas (BCC). Sixty percent had craniofacial resection alone, and 28.6% also had postoperative radiotherapy. There were two perioperative deaths, and 37.1% suffered early and 14.3% late surgical complications. Two- and five- year survival was significantly better (P = .02) with BCC (92% and 76%) than with SCC (54% and 24%). Long-term disease-specific survival was 20%, and 11.4% of our subjects were living with disease. Intracranial extension (P = .02), perineural invasion (P = .049), and prior radiotherapy significantly decreased 5-year survival. Conclusions: Acceptable mortality and morbidity is possible using craniofacial resection to treat advanced nonmelanoma skin cancer. Although disease-specific survival remains poor, positive trends were noted in local control beginning at 2 years of follow-up. Because patients often have few remaining options for cure, craniofacial resection is justified when technically feasible. [source]


The Concordance Index C and the Mann,Whitney Parameter Pr(X>Y) with Randomly Censored Data

BIOMETRICAL JOURNAL, Issue 3 2009
James A. Koziol
Abstract Harrell's c -index or concordance C has been widely used as a measure of separation of two survival distributions. In the absence of censored data, the c -index estimates the Mann,Whitney parameter Pr(X>Y), which has been repeatedly utilized in various statistical contexts. In the presence of randomly censored data, the c -index no longer estimates Pr(X>Y); rather, a parameter that involves the underlying censoring distributions. This is in contrast to Efron's maximum likelihood estimator of the Mann,Whitney parameter, which is recommended in the setting of random censorship. [source]


Cox Regression Methods for Two-Stage Randomization Designs

BIOMETRICS, Issue 2 2007
Yuliya Lokhnygina
Summary Two-stage randomization designs (TSRD) are becoming increasingly common in oncology and AIDS clinical trials as they make more efficient use of study participants to examine therapeutic regimens. In these designs patients are initially randomized to an induction treatment, followed by randomization to a maintenance treatment conditional on their induction response and consent to further study treatment. Broader acceptance of TSRDs in drug development may hinge on the ability to make appropriate intent-to-treat type inference within this design framework as to whether an experimental induction regimen is better than a standard induction regimen when maintenance treatment is fixed. Recently Lunceford, Davidian, and Tsiatis (2002, Biometrics58, 48,57) introduced an inverse probability weighting based analytical framework for estimating survival distributions and mean restricted survival times, as well as for comparing treatment policies at landmarks in the TSRD setting. In practice Cox regression is widely used and in this article we extend the analytical framework of Lunceford et al. (2002) to derive a consistent estimator for the log hazard in the Cox model and a robust score test to compare treatment policies. Large sample properties of these methods are derived, illustrated via a simulation study, and applied to a TSRD clinical trial. [source]


Survival Analysis in Clinical Trials: Past Developments and Future Directions

BIOMETRICS, Issue 4 2000
Thomas R. Fleming
Summary. The field of survival analysis emerged in the 20th century and experienced tremendous growth during the latter half of the century. The developments in this field that have had the most profound impact on clinical trials are the Kaplan-Meier (1958, Journal of the American Statistical Association53, 457,481) method for estimating the survival function, the log-rank statistic (Mantel, 1966, Cancer Chemotherapy Report50, 163,170) for comparing two survival distributions, and the Cox (1972, Journal of the Royal Statistical Society, Series B34, 187,220) proportional hazards model for quantifying the effects of covariates on the survival time. The counting-process martingale theory pioneered by Aalen (1975, Statistical inference for a family of counting processes, Ph.D. dissertation, University of California, Berkeley) provides a unified framework for studying the small- and large-sample properties of survival analysis statistics. Significant progress has been achieved and further developments are expected in many other areas, including the accelerated failure time model, multivariate failure time data, interval-censored data, dependent censoring, dynamic treatment regimes and causal inference, joint modeling of failure time and longitudinal data, and Baysian methods. [source]