Cyclically stable populations∗

The cyclically stable population relaxes the stable population assumption of fixed vital rates and replaces it with the assumption of a recurring sequence of schedules of vital rates. From any point (or stage) in one cycle of the sequence to the same stage in the next cycle, the cyclically stable population grows at a constant rate (λ). While the age composition of the cyclically stable population is different at different stages of the same cycle, it always has the same age composition at the same stage of every cycle. The essential dynamics of the cyclically stable model are captured by its birth projection matrix (BPM). The dominant eigenvalue of the BPM is growth rate A, and the right eigenvector associated with λ gives the within cycle‐birth sequence.

An important special case occurs when λ = 1, and a cyclically stationary population arises. Such populations challenge simplistic ideas about “Zero Population Growth.”; A population projection based on the sets of rates observed in the United States, 1970–90, shows a cyclically stationary population arising in less than 100 years. While it experiences no long term growth, that cyclically stationary population exhibits fluctuations in total size and considerable variability in age structure.     

On parameter estimation in Bernoulli trials with dependence

A simple estimator is proposed for the dependence parameter for the Klotz model of Bernoulli trials with Markov dependence and it is compared with the ratio estimator given by Price and the approximate maximum likelihood estimator given by Klotz. The proposed estimator is shown to have considerably smaller bias than the other two estimators with comparable mean squared errors, and has all the large sample optimal properties that the other two estimators have.     

The Tea Culture of China

Tihere is a Chinese old saying, ＂There are seven necessities of life, i.e., firewood, rice, cooking oil,salt, soy.sauce,vinegar and tea＂. You cannot miss any one in your daily life. It is clear that tea occupies an important position in Chinese people＇s life. In fact,     

Darfurian Livelihoods and Libya: Trade,Migration, and Remittance Flows in Times of Conflict and Crisis1

Labor migration and commerce between Sudan and Libya have long been features of livelihoods in Darfur. This paper describes the importance of historical trade and migration links between Darfur and Libya, and provides a background to the political and economic situation in Libya which has influenced opportunities for Sudanese migrant workers. A case study of the situation of the Darfurian migrants in Kufra (an oasis and transnational trade hub in southern Libya) illustrates how the recent Darfur conflict has affected migration patterns from Darfur and remittance flows in the opposite direction. Official estimates of Darfurian migrant workers in Libya were unavailable but were estimated to be between 150,000 and 250,000. The closure of the national border between Sudan and Libya in May 2003, largely a result of insecurity in Darfur, stopped the traffic of migrant workers between northern Darfur and southern Libya (which prevented the onward travel to Sudan of several thousand migrants in Kufra), and curtailed the well‐established trade routes, communications, and remittance flows. The current limited economic prospects for migrant workers in Libya, combined with the threat of detention, difficulties of return to Sudan, and loss of contact with and uncertainty about the fate of their families in Darfur, have created a sense of despair among many Darfurians. The paper concludes with a series of recommendations to improve the conditions of the Darfurian migrants in Libya, including an amnesty for illegal migrants, and also to ease the travel of migrants, promote communications between Libya and Darfur, and support the flow of remittances.     

Developmental Trends in Adolescent Suicidal Ideation

Research using retrospective or cross‐sectional data suggests that suicidal ideation rates peak during mid‐adolescence. This study used a longitudinal, community sample of adolescents who reported suicidal ideation repeatedly over 7 years to examine suicidal ideation trends. We hypothesized that a mid‐adolescence peak in ideation is limited to adolescents exposed to friends' or family members' suicidality. Ideation trends were compared across groups exposed and not exposed to others' suicidality and statistically assessed using growth curve analyses. Overall, ideation rates peaked at age 15 years. Group comparisons revealed that the peak was limited to adolescents whose parents reported suicidality.     

Space–time calibration of radar rainfall data

Motivated by a specific problem concerning the relationship between radar reflectance and rainfall intensity, the paper develops a space–time model for use in environmental monitoring applications. The model is cast as a high dimensional multivariate state space time series model, in which the cross-covariance structure is derived from the spatial context of the component series, in such a way that its interpretation is essentially independent of the particular set of spatial locations at which the data are recorded. We develop algorithms for estimating the parameters of the model by maximum likelihood, and for making spatial predictions of the radar calibration parameters by using realtime computations. We apply the model to data from a weather radar station in Lancashire, England, and demonstrate through empirical validation the predictive performance of the model.     

The soil seed bank and its relationship to the aboveground vegetation in deciduous forests in New York City

The soil seed bank was studied in two deciduous forests in Bronx (New York City), NY. The purpose of this study was to determine how the biotic andabiotic differences between urban and rural forests arereflected in urban forest seed banks. Soil samples werecollected in two consecutive years and monitored for emergencein the greenhouse over two years. In 1993, the mean number ofemergents ranged from 4636 to 5373 m^-2 (excluding ferns), or from 6972 to 9651 m^-2 (including ferns). In 1994, the mean number of emergents ranged from 1656 to 2013 m^-2 (excluding ferns), or from 5019 to 5992 m^-2 (including ferns). Graminoids and fernscombined accounted for approximately 70% of all emergents eachyear. Three taxa, Rubus spp., Betula lenta, and Liriodendron tulipifera, comprised 60–80% of the woody emergents and were theonly woody taxa to exhibit delayed germination. A substantialnumber of forbs, graminoids and ferns (15–50%) exhibiteddelayed germination. The nonnative woody species Ailanthusaltissima, Morus alba, and Celastrus orbiculatus were absentfrom the aboveground vegetation of some forest plots yet werepresent at low densities in the seed bank. A greater meandensity of emergents and the presence of nonnative species arethe main differences between the seed banks of these urbanforests and those reported for similar nonurban forests in thisregion.     

The Forgotten Siblings

In this article I wish to argue that the qualities and dynamics of sibling relationships may have been overlooked in family therapy. In overlooking them, we have also overlooked a significant feature in the emotional life of children. I would like to suggest that sibling relationships are where children practise identity. In these relationships we can learn how to be one in a group. Family Therapy treatment may not make enough use of the dynamic of the sibling relationship.