Survival analysis is an effective tool for conservation studies, since it measure the risk of an event that is important for the survival of populations and preservation of biodiversity. In this thesis three different models for survival analysis are used to estimate the age at breeding between two genetically different populations of brown trout. These populations are an evolutionary enigma, since they apparently coexist in direct competition with each other, which according to ecological theory should not happen. Thus it is of interest if differences between them can be identified. The data consists of brown trouts and has been collected over 20 years. The models are the Cox Proportional Hazards model, the Complementary Log-Log Link model and the Log Logistic Accelerated Failure-Time model. The Cox model were estimated in three different ways due to the nonproportional hazards in the estimates of time to breeding, which gave different interpretations of the same model. All of the models agree that the population B breed at younger ages than the population A, which suggests that the two populations have different reproductive strategies.

Objetivo. Uma coorte de crianças foi observada sob duas abordagens: coorte prospectiva e coorte retrospectiva (histórica) e o objetivo foi comparar as estimativas da função de riscos do modelo de Cox entre as duas abordagens e a mesma comparação com o modelo complemento log-log. Além disso, comparar as estimativas da função de riscos pelos modelos de Cox e complemento log-log para cada uma das abordagens, separadamente. Métodos. Quando se estuda o tempo de sobrevida da amamentação sem as covariáveis, foram obtidas as estimativas pela técnica atuarial e modelo complemento log-log da informação retrospectiva e a técnica de Kaplan-Meier para a informação prospectiva. Os modelos de Cox e complemento log-log foram utilizados para estimar a razão de riscos (HR) com covariáveis para as duas abordagens. Resultados. Sem as covariáveis, a comparação entre as duas abordagens mostrou que as estimativas de S(t) pela informação retrospectiva estão um pouco defasadas em relação à medida prospectiva. Com as covariáveis, os resultados dos modelos de Cox e complemento log-log são semelhantes tanto para os dados prospectivos quanto para os retrospectivos. Foram semelhantes, também, os resultados de um mesmo modelo para cada uma das fontes de informação. Conclusões. Em geral, as estimativas foram bastante próximas em quaisquer das comparações. Somente a variável hábito de fumar da mãe durante a gravidez permaneceu nos modelos finais para todas as técnicas utilizadas, com estimativas próximas, reforçando semelhança entre as várias abordagens. / Objective. A cohort of children was observed under two approaches: prospective cohort and retrospective (historical) cohort and the objective was to compare the estimates of the hazard ratio from Cox model between the two approaches and do the same comparison using the complementary log-log model. Moreover, to compare the estimates of the hazard ratio from the two models to each one of the approaches, separately. Methods. When the survival time for breastfeeding is studied without the covariates, the comparison between the results of actuarial technique and the complementary log-log model has been made for the recorded information. The Kaplan-Meier technique has been used with the daily notebook measures. The Cox and complementary log-log models can estimate the risk rate of covariates categories to both approaches. Results. Without the covariates, the comparison between the two information resources showed that retrospective measures give lower estimates than that from the prospective measures. With covariates, the estimates are not so different and led to the same results. Conclusions. The estimates of each one of the comparisons were too close. Just the variable mothers smoking during the pregnancy stayed in the final models for every techniques used, with close estimates, reinforcing likeness between the several approaches.

Despite the Family Code of 2000 that raised the legal age at marriage to 18 for both sexes; early family formation is still a common practice which affect many children in Ethiopia. Previous research has shown that girls in rural areas are more disadvantaged and suffer the consequences of early marriage the most. The purpose of this thesis is to study the risk factors for early marriage for both girls and boys in an urban area, Shashemene. Also, using longitudinal data, trends in early family formation between 1973 and 2008 have been analyzed with an event history approach. The data were collected as part of the project “Changing Ethiopia: Urban livelihood, gender, and ethnicity in Shashemene after 35 years: A case study”. The discrete-time complementary log-log regression estimates have provided evidence of gender inequality in early marriage formations, showing that girls are more prone to experience early marriage than boys. While area of birth (rural-urban) has no direct impact on the risk of early marriage, it is found that living in an urban area offsets the effect of area of birth, suggesting a selection process into migration. It is also found that school attendance decreases the likelihood of early marriage, while literacy has little effect. Moreover, among people living in Shashemene, religious affiliation has more impact on early marriage risks than ethnic identity and the first language. Finally, there was little evidence on period and cohort effects for early family formation.

Objetivo. Uma coorte de crianças foi observada sob duas abordagens: coorte prospectiva e coorte retrospectiva (histórica) e o objetivo foi comparar as estimativas da função de riscos do modelo de Cox entre as duas abordagens e a mesma comparação com o modelo complemento log-log. Além disso, comparar as estimativas da função de riscos pelos modelos de Cox e complemento log-log para cada uma das abordagens, separadamente. Métodos. Quando se estuda o tempo de sobrevida da amamentação sem as covariáveis, foram obtidas as estimativas pela técnica atuarial e modelo complemento log-log da informação retrospectiva e a técnica de Kaplan-Meier para a informação prospectiva. Os modelos de Cox e complemento log-log foram utilizados para estimar a razão de riscos (HR) com covariáveis para as duas abordagens. Resultados. Sem as covariáveis, a comparação entre as duas abordagens mostrou que as estimativas de S(t) pela informação retrospectiva estão um pouco defasadas em relação à medida prospectiva. Com as covariáveis, os resultados dos modelos de Cox e complemento log-log são semelhantes tanto para os dados prospectivos quanto para os retrospectivos. Foram semelhantes, também, os resultados de um mesmo modelo para cada uma das fontes de informação. Conclusões. Em geral, as estimativas foram bastante próximas em quaisquer das comparações. Somente a variável hábito de fumar da mãe durante a gravidez permaneceu nos modelos finais para todas as técnicas utilizadas, com estimativas próximas, reforçando semelhança entre as várias abordagens. / Objective. A cohort of children was observed under two approaches: prospective cohort and retrospective (historical) cohort and the objective was to compare the estimates of the hazard ratio from Cox model between the two approaches and do the same comparison using the complementary log-log model. Moreover, to compare the estimates of the hazard ratio from the two models to each one of the approaches, separately. Methods. When the survival time for breastfeeding is studied without the covariates, the comparison between the results of actuarial technique and the complementary log-log model has been made for the recorded information. The Kaplan-Meier technique has been used with the daily notebook measures. The Cox and complementary log-log models can estimate the risk rate of covariates categories to both approaches. Results. Without the covariates, the comparison between the two information resources showed that retrospective measures give lower estimates than that from the prospective measures. With covariates, the estimates are not so different and led to the same results. Conclusions. The estimates of each one of the comparisons were too close. Just the variable mothers smoking during the pregnancy stayed in the final models for every techniques used, with close estimates, reinforcing likeness between the several approaches.