A method for checking regression models in survival analysis based on the risk score

We propose to perform model check for the Cox and Aalen regression models using martingale residual processes grouped after the risk score. Asymptotic distributions of the grouped martingale residual processes are deduced, so both formal and graphical model check can be performed. The method is validated by stochastic simulation. A data example with patients with primary biliary cirrhosis of the liver is discussed.     

Main and Interaction Effects of Women's Education and Status on Fertility: The Case of Tanzania

The analysis is based on individual 1996 TDHS data combined withaggregate data from the 1988 census and the 1991/1992 TDHS. When varioussources of spuriousness are taken into account, it is found that giving awoman more education reduces her fertility much less than suggested byunivariate tabulations of the total fertility rate. Expansion of primaryeducation contributes to only a slightly higher age at first birth, and theeffect on higher-order birth rates is not significant. Changes in post-partum insusceptibility outweigh those in fertility desires and use ofmodern contraception among women not wanting an additional child.Secondary school enrollment influences fertility more markedly, inparticular because of a later first birth. Effects of women's status areestimated in models for actual fertility as well as fertility desires, post-partum insusceptibility and contraceptive use, using up to six macro- ormicro-level indicators. All significant effects suggest that empowerment ofwomen will tend to push fertility down, net of education. The significantinteractions between women's status and education point in differentdirections, but a majority of them indicate that education has the mostpronounced effect on fertility in the more egalitarian regions and amongwomen with relatively high individual status.     

Detection of biomass change in a Norwegian mountain forest area using small footprint airborne laser scanner data

Different approaches for estimation of change in biomass between two points in time by means of airborne laser scanner data were tested. Both field and laser data were collected at two occasions on 52 sample plots in a mountain forest in southeastern Norway. In the first approach, biomass change was estimated as the difference between predicted biomass for the two measurement occasions. Joint models for the biomass at both occasions were fitted using different height and density variables from laser data as explanatory variables. The second approach modelled the observed change directly using the change in different variables extracted from the laser data as explanatory variables. In the third approach we modelled the relative change in biomass. The explanatory variables were also expressed as relative change between measurement occasions. In all approaches we allowed spline terms to be entered. We also investigated the aptness of models for which the residual variance was modeled by allowing it to be proportional to the area of the plot on which biomass was assessed. All alternative models were initially assessed by AIC. All models were also evaluated by estimating biomass change on the model development data. This evaluation indicated that the two direct approaches (approach 2 and 3) were better than relying on modeling biomass at both occasions and taking change as the difference between biomass estimates. Approach 2 seemed to be slightly better than approach 3 based on assessments of bias in the evaluation.     

The Current Duration Approach to Estimating Time to Pregnancy

Abstract. Time‐to‐pregnancy (TTP) is the duration from the time a couple starts trying to become pregnant until they succeed. It is considered one of the most direct methods to measure natural fecundity in humans. Statistical tools for designing and analysing time to pregnancy studies belong to survival analysis, but several features require special attention. Prospective designs are difficult to carry out and retrospective (pregnancy‐based) designs, being widely used in this area, do not allow efficiently including couples remaining childless. A third possible design starts from a cross‐sectional sample of couples currently trying to become pregnant, using current duration (backward recurrence time) as basis for the estimation of TTP. Regression analysis is then most conveniently carried out in the accelerated failure time model. This paper surveys some practical and technical‐statistical issues in implementing this approach in a large telephone‐based survey, the Epidemiological Observatory of Fecundity in France (Obseff).     

Nigerians in China: A Second State of Immobility

China’s rapid economic development has been accompanied by new forms of immigration. Investors and professionals from developed countries are increasingly joined by a diverse group of immigrants from around the world. While there is a large body of academic literature on Chinese emigration, China’s new role as a country of immigration has received less scholarly attention. This paper addresses the dynamics of South–South migration to China through a study of Nigerians in Guangzhou, a major international trading hub. The analysis is based on qualitative interviews and participant observation among African traders and migrants in Guangzhou. The paper contends that Nigerian immigration to China epitomizes global migration trends towards a diversification of migration flows, commercialization of the migration process and increased policing of foreigners within national borders. China was rarely the preferred destination of this study’s Nigerian informants but, rather, a palatable alternative, as their aspirations to enter Europe and North America were curtailed by restrictive immigration regimes. They escaped a situation of involuntary immobility in Nigeria through short‐term visas obtained with the help of migration brokers. However, opportunities for visa renewals are scant under the current Chinese immigration policy. Undocumented migrants find their mobility severely inhibited: They must carefully assess how, when and with whom they move about in order to avoid police interception. This is a business impediment, as well as a source of personal distress for migrants who engage in trade and the provision of trade‐related services. The situation can be described as a “second state of immobility”: the migrants have succeeded in the difficult project of emigration, but find themselves spatially entrapped in new ways in their destination country.     

A Sequential Point Process Model and Bayesian Inference for Spatial Point Patterns with Linear Structures

Abstract. We introduce a flexible spatial point process model for spatial point patterns exhibiting linear structures, without incorporating a latent line process. The model is given by an underlying sequential point process model. Under this model, the points can be of one of three types: a ‘background point’ an ‘independent cluster point’ or a ‘dependent cluster point’. The background and independent cluster points are thought to exhibit ‘complete spatial randomness’, whereas the dependent cluster points are likely to occur close to previous cluster points. We demonstrate the flexibility of the model for producing point patterns with linear structures and propose to use the model as the likelihood in a Bayesian setting when analysing a spatial point pattern exhibiting linear structures. We illustrate this methodology by analysing two spatial point pattern datasets (locations of bronze age graves in Denmark and locations of mountain tops in Spain).