Article

Understanding survival analysis: Kaplan-Meier estimate

Department of Community Medicine, Post Graduate Institute of Medical Science, Rohtak, Haryana, India.
10/2010; 1(4):274-8. DOI: 10.4103/0974-7788.76794
Source: PubMed

ABSTRACT

Kaplan-Meier estimate is one of the best options to be used to measure the fraction of subjects living for a certain amount of time after treatment. In clinical trials or community trials, the effect of an intervention is assessed by measuring the number of subjects survived or saved after that intervention over a period of time. The time starting from a defined point to the occurrence of a given event, for example death is called as survival time and the analysis of group data as survival analysis. This can be affected by subjects under study that are uncooperative and refused to be remained in the study or when some of the subjects may not experience the event or death before the end of the study, although they would have experienced or died if observation continued, or we lose touch with them midway in the study. We label these situations as censored observations. The Kaplan-Meier estimate is the simplest way of computing the survival over time in spite of all these difficulties associated with subjects or situations. The survival curve can be created assuming various situations. It involves computing of probabilities of occurrence of event at a certain point of time and multiplying these successive probabilities by any earlier computed probabilities to get the final estimate. This can be calculated for two groups of subjects and also their statistical difference in the survivals. This can be used in Ayurveda research when they are comparing two drugs and looking for survival of subjects.

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• "where T is the renewal rate (y −1 ); MLS is the median lifespan (d) We used the non-parametric Kaplan-Meier method to determine the survivorship rate (S) of roots that were growing over a given period of time (Majdi et al. 2001; Tierney and Fahey 2001). as well as the MLS (Goel et al. 2010; Crawley 2012). Each individual root had an equal weight and was classified as still live (censored, not dead yet) or dead (uncensored) at the end of the study. "
Article: Unexpected phenology and lifespan of shallow and deep fine roots of walnut trees grown in a silvoarable Mediterranean agroforestry system
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ABSTRACT: Background and Aims: Fine roots play a major role in the global carbon cycle through respiration, exudation and decomposition processes, but their dynamics are poorly understood. Current estimates of root dynamics have principally been observed in shallow soil horizons (<1 m), and mainly in forest systems.We studied walnut (Juglans regia×nigra L.) finerootdynamicsinanag- roforestry system in a Mediterranean climate, with a focus on deep soils (down to 5 m), and root dynamics throughout the year. Methods: Sixteenminirhizotron tubes were installed in a soil pit, at depths of 0.0–0.7, 1.0–1.7, 2.5–3.2 and 4.0– 4.7 m and at two distances from the nearest trees (2 and 5m). Fine root (diameter≤2mm) dynamicswere record- ed across three diameter classes every 3weeks for 1 year to determine their phenology and turnover in relation to soil depth, root diameter and distance from the tree row. Results: Deep (>2.5 m) root growth occurred at two distinct periods, at bud break in spring and throughout the winter i.e., after leaf fall. In contrast, shallow roots grew mainly during the spring-summer period. Maximum root elongation rates ranged from 1 to 2cmday−1 depending on soil depth.Most rootmortality occurred in upper soil layers whereas only 10% of fine roots below 4 m died over the study period. Fine root lifespan was longer in thicker and in deeper roots with the lifespan of the thinnest roots (0.0–0.5 mm) increas- ing from 129 days in the topsoil to 190 at depths>2.5 m Conclusions: The unexpected growth of very deep fine roots during the winter months, which is unusual for a deciduous tree species, suggests that deep and shallow roots share different physiological strategies and that current estimates based on the shortest root growth periods (i.e., during spring and summer) may be underestimating root production. Although high fine root turnover rates might partially result from the minirhizotron approach used, our results help gain in- sight into some of the factors driving soil organic carbon content.
Full-text · Article · Dec 2015 · Plant and Soil
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• "The age of first mating was determined using the Kaplan-Meier survival analysis with the R package 'survival'. Survival analysis is a technique for analyzing 'time-to-event' or 'failure-time' data, which may or may not be related to survival and death in the usual sense (Goel et al. 2010). In calculating a Kaplan-Meier curve, the probability of occurrence of the first mating was obtained from the proportion of at-risk C. luridus adults that performed the first mating. "
Article: Mating Behavior and Evidence for Male-Produced Aggregation Pheromone in Cyrtomon luridus (Boheman) (Coleoptera: Curculionidae: Entiminae)
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ABSTRACT: Cyrtomon luridus (Boheman) (Coleoptera: Curculionidae: Entiminae) is a serious pest of Duboisia sp. (Solanaceae) in Brazil. However, our knowledge of the reproductive behavior and the chemical ecology of this curculionid is based on very limited data. The aim of this study was therefore to describe the mating behavior of C. luridus and the olfactory response of males and females to volatiles of conspecifics alone or conspecifics with a host plant. The mating behavior was observed in newly emerged pairs for 9 days in the laboratory. A Y-tube olfactometer was used to evaluate the responses of beetles to odor sources. The paired adults began to mate 2 days after emergence, and they displayed repeated matings that increased in frequency with age. The males were the most active sex in the mating system of C. luridus. The mating sequence in this species was divided into precopulatory, copulatory and postcopulatory phases. In the olfactometer bioassays, the volatiles emitted from males that fed on Duboisia sp. were attractive to conspecific weevils. The evidence from this result indicates that chemical communication in C. luridus is likely mediated by an aggrega-tion pheromone produced by males. Our findings provide new insights into the mating system and chemical ecology of C. luridus and may serve as a foundation for future studies to develop strategies for the management of this curculionid in the field.
Full-text · Article · Jan 2015 · Journal of Insect Behavior
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• "By convention, the Kaplan-Meier plots are represented with steps to indicate the time in which the terminal events (symptoms) occur and signs (+) to indicate censored observations (Carvalho et al., 2011). The non-parametric approach of the survival analysis using the Kaplan-Meier method allows statistical significance tests to compare treatments (Akbar et al., 2009;Goel et al., 2010), such as cultivars and fruit inoculation times addressed in this study. In this context, the most used test in survival analysis is the log-rank test, but should only be applied to compare groups defined by categorical variables (Akbar et al., 2009) and when the ratio of hazard functions of the compared treatments is approximately constant, which is called proportional hazards. "
Article: Survival analysis: A tool in the study of post-harvest diseases in peaches
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ABSTRACT: A análise de sobrevivência é aplicada quando o tempo até a ocorrência de um evento for o objeto de interesse. Em doenças de plantas, dados dessa natureza são rotineiramente coletados, embora aplicações do método sejam pouco comuns. O objetivo deste trabalho foi utilizar dois estudos de doenças em pós-colheita de pêssegos, considerando-se safras conjuntamente e a existência de efeito aleatório, compartilhado por frutos de uma mesma árvore, para descrever as principais técnicas em análise de sobrevivência. Aplicaram-se a técnica não paramétrica de Kaplan-Meier e a estatística log-rank, além do modelo semiparamétrico de riscos proporcionais, de Cox, para estimar o efeito de cultivares e do número de dias após a floração plena sobre a sobrevivência ao sintoma de podridão parda e sobre o risco instantâneo de expressá-lo, em duas safras consecutivas. A análise conjunta com efeito basal, variando entre safras, e a verificação do efeito de árvore como fator de agrupamento com efeito aleatório, mostraram-se adequadas para interpretar o fenômeno avaliado (doença) e podem ser ferramentas importantes para substituir ou complementar as análises convencionais, respeitando-se as naturezas da variável e do fenômeno.
Full-text · Article · Jan 2015 · Revista Ceres
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