Fecundability,coital frequency and the viability of Ova

An extension of Barrett and Marshall's model expressing fecundability as a function of coital pattern is proposed. In particular, this extension includes the probability that the ovum remains alive. The extended model has been applied to Barrett and Marshall's data, a series of cycles for which basal body temperature curves and the date of coitus have been recorded. It was thus possible to estimate the daily probabilities of fertilization and, under certain assumptions, the proportion of lost ova. This proportion was estimated to be above 50 per cent, and increases with age. The increase of fecundability with frequency of intercourse is more moderate than that predicted by Barrett and Marshall's model.     

Fecundability, coital frequency and the viability of Ova

Summary An extension of Barrett and Marshall's model expressing fecundability as a function of coital pattern is proposed. In particular, this extension includes the probability that the ovum remains alive. The extended model has been applied to Barrett and Marshall's data, a series of cycles for which basal body temperature curves and the date of coitus have been recorded. It was thus possible to estimate the daily probabilities of fertilization and, under certain assumptions, the proportion of lost ova. This proportion was estimated to be above 50 per cent, and increases with age. The increase of fecundability with frequency of intercourse is more moderate than that predicted by Barrett and Marshall's model.     

Grouped zero-inflated count data models of coital frequency

Various count data models are applied to data collected from a sample of Norfolk young persons, who were asked how many times they had had sexual intercourse during the previous two-week period. The models take account of the fact that the data are “grouped”, meaning that for some observations, the count is not known exactly but is known to fall in a particular range. Using a formal testing procedure, we find overwhelming evidence of the presence of excess zeros, and this we attribute to the fact that, at any time, a certain proportion of the population are sexually inactive. Our final model contains two equations, the first being the participation equation which determines whether an individual is sexually active, and the second being the frequency equation which determines the count, conditional on being active. Age, gender, salary, occupational status, marital status and type of living environment all have interesting effects on either participation or frequency. Since a significant proportion of the original sample declined to reveal coital frequency, we address the potential problem of selection bias by including a Heckman-type correction term during the model selection process. Received: 27 November 1998/Accepted: 19 May 1999     

A Model of Age-specific Fecundability

A new model of the behavioural and physiological causes of age-specific variation in marital fecundability is presented. Total fecundability is decomposed into a series of susceptibility factors (the length of ovarian cycles, the length of the fertile period within each cycle, the probability that a cycle is ovulatory, and the likelihood that an act of unprotected intercourse within the fertile period results in conception) and an exposure factor reflecting the effect of duration of marriage on coital frequency. The impact of intra-uterine mortality on effective fecundability is also modelled. Data on western women, from which standard age curves of fecundability are estimated, suggest that any decline in fecundity between ages 30 and 40 is attributable to changes, not in the ability to conceive, but in the capacity to carry a pregnancy to term. Sensitivity tests suggest that the most important potential sources of inter-population variation in fecundability are intra-uterine death and the incidence of anovulatory cycles.     

Pregnancy avoidance and coital behavior

This study proposes a behavioral model that identifies determinants of coital activity in the context of pregnancy avoidance and assesses the relationships using weekly panel data collected on 300 rural married women in Sri Lanka in 1988. We discuss the utility of the design, which is similar to that of an epidemiological surveillance system, for the measurement of coital behavior and pregnancy risk perceptions. Perceptions of pregnancy risk, spousal agreement on sexual relations, menstrual and lactational status, and cycle timing, all measured daily, are found to influence significantly the probability of coitus on that day. The findings suggest that substantial gains in studying fertility regulation are likely from closer investigation of the behavioral connections between motivation for pregnancy avoidance and coital incidence. Comparison of such panel data with those of cross-sectional sample surveys also provides insights into the validity of measures of coital and contraceptive behavior.     

Modeling Seasonality in Fecundability,Conceptions, and Births

This paper develops a model of seasonal fluctuations in fecundability, conceptions, and births. We begin with a model of individual fecundability that combines behavioral and biological components, with particular attention to the roles of coital frequency, sperm concentration, fetal loss, and contraception. The individual-level model is then expanded into a model of seasonal fluctuations in births at the population level, which accounts explicitly for seasonal fluctuations in the size of the susceptible population. We illustrate the use of the model by analyzing proposed explanations of birth seasonality that rely on extreme summer heat.     

Fecundability and its relation to age in a sample of Taiwanese women

Abstract In this paper reliable estimates of mean fecundability at marriage for a sample of heterogeneous Taiwanese women are obtained by using a set of data collected retrospectively. The effects of 'truncation bias' and 'memory bias' are estimated by studying the relationships between mean fecundability and the duration of marriage. Then the variations in fecundabilities by age at marriage are studied. The data are taken from an intensive fertility survey of married women between the ages of 20 and 39 conducted in Taichung city of Taiwan towards the end of 1962 prior to a year-long family planning action programme. Fecundability, the probability of conception in the absence of contraception, is estimated by using a Type I geometric model and is estimated from the observed distribution of first pregnancy intervals-the period between the onset of marriage and the beginning of first conception. The estimated fecundability level relatively free from truncation and memory bias is 195±3 per 1.000 women. The fecundability level increases with wife's age at marriage (up to 25) independently of its association with the duration of marriage.     

Fecundability and the Frequency of Marital Intercourse: A Critique of Nine Models

The biological and demographic literature on the effects or frequency of intercourse on fecundability is reviewed in this paper. While empirical work and model-building results agree well on the effect of change from low to moderate coital frequencies, inconsistencies emerge when increase from moderate to high coital frequencies is considered. Of the models considered, it may be argued that the explicit provision for unfavourable cycles makes Schwartz's model an improvement over Barrett and Marshall's, Lachenbruch's and Trussell's. Moreover, even though he used a very different approach, Bongaarts obtained results that are much closer to those of Schwartz et al. than the others. Bongaarts's model is used as a starting point for new modelling, to be reported in the next issue, which deals with these inconsistencies by taking into account the ageing of gametes.     

Fecundability and the Frequency of Marital Intercourse: New Models Incorporating the Ageing of Gametes

A companion paper in the last issue reviewed the literature on effects of frequency of intercourse on fecundability. At moderately high coital frequencies, predicted effects of further increases on fecundability based on previous work vary widely. New modelling reported here, designed to take into account the ageing of gametes, attempts to identify a set of limits within which the true relationship of coital frequency and fecundability at moderate to high frequencies may confidently be expected to lie.     

The Effects of Breastfeeding and Nutrition on Fecundability in Rural Bangladesh: A Hazards-Model Analysis

Data taken from the ‘Determinants of Natural Fertility’ study, Bangladesh, are used with multivariate hazard models to study variations in fecundability between women, especially the relationship between nutritional status, breastfeeding practices, and the monthly probability of conceiving. It is found that fecundability varies both between women of a given age and, for a particular woman, by age. The variation is related to four variables: separation, which affects coital frequency; age, which represents biological changes; lactation practices; and the duration of amenorrhoea. Nutrition above famine or starvation levels is not a significant determinant of fecundability. The most surprising finding is the effect of the pattern of breastfeeding on fecundability: as a menstruating woman begins to wean her child, her probability of conceiving increases as her serum prolactin, which inhibits ovulation, decreases. Thus, those menstruating women who are most likely to conceive are those who have completely weaned their infants in the very recent past.     

Estimation of stage-specific developmental times and survivorship from stage frequency data

Summary An algorithm for estimating mean stage durations, the standard deviations associated with the means, and stage-specific survivorships from stage frequency data is presented. The algorithm is based on an age-structured, distributed delay simulation model which usesErlang distributions to determine the probability of maturity for individuals in each stage. If the data set extends beyond the first generation, the algorithm requires a fecundity rate, as well as stage frequencies, as input. Goodness-of-fit was measured using a weighted least squares calculation summed over all observed stages and all sampling dates.     

Fetal movement frequency and the effect of associated perinatal factors: Multicenter study

Background

Decreased fetal movements are associated with adverse perinatal outcomes, including stillbirth. Delayed maternal visits to a health care provider after perceiving decreased fetal movements are frequently observed in stillbirths. Informing pregnant women of the normal range of fetal movement frequency is essential in their earlier visits in order to prevent stillbirth.

The association between the frequency of wife assault and marital dissolution in Canada

A strong positive association between wife abuse in the first marriage and the probability of that marriage ending is documented and investigated using the 1993 Violence Against Women survey (VAWS) for Canada and controlling for the endogeneity of abuse. A sensitivity analysis suggests that systematic reporting differences are not likely to account for the findings. Received: 2 June 1999/Accepted: 13 June 2000     

Evaluation of statistical precision and design of efficient sampling for the population estimation based on frequency of occurrence

Summary The binomial sampling to estimate population density of an organism based simply upon the frequency of its occurrence among sampled quadrats is a labour-saving technique which is potentially useful for small animals like insects and has actually been applied occasionally to studies of their populations. The present study provides a theoretical basis for this convenient technique, which makes it statistically reliable and tolerable for consistent use in intensive as well as preliminary population censuses. Firs, the magnitude of sampling error in relation to sample size is formulated mathematically for the estimate to be obtained by this indirect method of census, using either of the two popular models relating frequency of occurrence (p) to mean density (m), i.e. the negative binomial model,p=1−(1+m/k) ^−k, and the empirical model,p=1−exp(−am ^b). Then, the equations to calculate sample size and census cost that are necessary to attain a given desired level of precision in the estimation are derived for both models. A notable feature of the relationship of necessary sample size (or census cost) to mean density in the frequency method, in constrast to that in the ordinary census, is that it shows a concave curve which tends to rise sharply not only towards lower but also towards higher levels of density. These theoretical results make it also possible to design sequential estimation procedures based on this convenient census technique, which may enable us with the least necessary cost to get a series of population estimates with the desired precision level. Examples are presented to explain how to apply these programs to acutal censuses in the field.     

Measuring the effect of changing legislation on the frequency of divorce: The Netherlands, 1830–1990

This article discusses different procedures for measuring the effects of judicial changes on divorce rates. It presents an alternative model and applies it to a historical time series for the Netherlands. In this model, intervention variables were added to a statistical time-series (ARIMA) model. The conclusion of our analysis was that the effects of three judicial changes were only temporary. Revised text of a paper presented at the seminar “The Role of the State and of Public Opinion in Sexual Attitudes and Demographic Behaviour since the 18th Century,” 17th International Congress of Historical Sciences/ Commission Internationale de Démographie Historique, held in Madrid, August 31-September 1, 1990. The views expressed in this paper are those of the authors and do not necessarily reflect the policies of the Netherlands Central Bureau of Statistics.     

RESEARCH NOTE Some evidence against the assumption that approval of family planning is associated with frequency of spouses' discussion of the subject

Much previous work on the relationship between respondent's reported frequency of discussion with spouse about family planning and correctness of reporting spouse's approval of family planning has led to the conclusion that discussion promotes approval. In this paper, data from the 1998 Kenya Demographic and Health Surveys are used to show that a focus on the relationship between frequency of discussion and correctness of reporting partner's disapproval of family planning leads to sceptical conclusions about the effects of discussion in improving knowledge of partner's attitude or in promoting approval.     

"Mummies and babies" and friends and lovers in Lesotho

This paper examines an institutionalized friendship among adolescent girls and young women in southern Africa. Lesotho's economy is based on migrant male labor which leaves the women dependent on male earnings or subsistence from the land, and also creates unstable marital relations. Young girls in the modern schools develop close relationships, called "mummy-baby," with slightly older girls. Sexual intimacy is an important aspect of these relationships. Mummy-baby relationships not only provide emotional support prior to marriage, but also a network of support for married and unmarried women in new towns or schools, either replacing or accompanying heterosexual bonds.     

Understanding and Using Mediators and Moderators

Mediation and moderation are two theories for refining and understanding a causal relationship. Empirical investigation of mediators and moderators requires an integrated research design rather than the data analyses driven approach often seen in the literature. This paper described the conceptual foundation, research design, data analysis, as well as inferences involved in a mediation and/or moderation investigation in both experimental and non-experimental (i.e., correlational) contexts. The essential distinctions between the investigation of mediators and moderators were summarized and juxtaposed in an example of a causal relationship between test difficulty and test anxiety. In addition, the more elaborate models, moderated mediation and mediated moderation, the use of structural equation models, and the problems with model misspecification were discussed conceptually.