[Intermediate variables in the relation between infant-child mortality and the country's educational level].
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Americas↗
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The fertility-inhibiting effects of the intermediate fertility variables.
Based on the application of an aggregate reproductive model, this study demonstrates that a small number of intermediate fertility variables are responsible for most of the variation in fertility levels of populations. Four variables--proportion married, contraception, induced abortion, and postpartum infecundability--are generally the most important determinants of fertility; the other intermediate factors are of less interest except in unusual circumstances. These four factors explain 96 percent of the variance in the total fertility rate in a sample of 41 populations that include developing and developed countries as well as historical populations. In the last section, the average fertility effect of the principal intermediate fertility variables is estimated for groups of contemporary populations with different total fertility rates.
Abortion, Induced↗
Fertility inhibiting effects of the intermediate fertility variables in two Nigerian sub-groups.
"In this paper, the fertility-inhibiting effects of the intermediate fertility variables among the Ikale-Yoruba are contrasted against those of the Ekiti, another Yoruba group for which more conventional family building practices have been reported. The objectives of the comparative approach are as follows: (a) to show that abstinence is not a factor in the reduction of marital fertility among the Ikale-Yoruba, as it is in other sub-groups studied so far; (b) that alternative outlets of fertility reduction, such as reliance on the protection offered by post-partum amenorrhoea, and some practice of folk and modern methods of contraception, are employed." The data, which were collected in Nigeria in 1977, concern 460 currently married Ikale-Yoruba women and some of their husbands, and 535 eligible Ekiti-Yoruba women. The results show that although total fertility rates between the two groups were similar, there were significant differences concerning the relative reliance by each group on postpartum abstinence and contraception. (summary in FRE, ITA)
Africa↗
An analysis of intermediate fertility variables in Shanghai, Hebei and Shaanxi.
"On the basis of the data from China's first thorough survey on fertility [conducted in 1985], organized by the State Statistical Bureau, this study will analyze...intermediate fertility variables in Shanghai, Hebei and Shaanxi according to the [Bongaarts] Model. It will also provide a general survey of the model."
Asia↗
Negative bias in exposure-response trends in occupational studies: modeling the healthy workers survivor effect.
Many occupational studies analyze trends between cumulative exposure and mortality. The authors show that such trends are, in general, negatively confounded by employment status. Mortality rates for workers who leave work ("inactive" workers) are higher than for active workers because some workers leave because they are ill. The percentage of inactive relative to active person-time is higher in low categories of cumulative exposure, causing employment status to act as a negative confounder of exposure-response trends (the opposite occurs for time-since-hire). We illustrate these phenomena using 10 "negative" mortality studies, in which adjustment for employment status removes false trends. However, adjustment for employment status will lead to biased estimates when it acts as an intermediate variable between cumulative exposure and death, as occurs directly when exposure causes a disabling disease that, in turn, causes death or indirectly when exposure causes workers to leave work. The authors illustrate this problem using simulated follow-up data for leaving, disease incidence, and mortality. In the null case in which cumulative exposure affects neither disease incidence (or mortality) nor leaving rates, employment status indeed acts as a negative confounder of exposure-response trends, and traditional adjustment eliminates this confounding. However, when cumulative exposure affects disease incidence or rates of leaving, adjustment for employment status will not be adequate. Employment status falls under the general rubric of variables that are simultaneously confounders and intermediate variables.
Bias↗
Identifiability and exchangeability for direct and indirect effects.
We consider the problem of separating the direct effects of an exposure from effects relayed through an intermediate variable (indirect effects). We show that adjustment for the intermediate variable, which is the most common method of estimating direct effects, can be biased. We also show that even in a randomized crossover trial of exposure, direct and indirect effects cannot be separated without special assumptions; in other words, direct and indirect effects are not separately identifiable when only exposure is randomized. If the exposure and intermediate never interact to cause disease and if intermediate effects can be controlled, that is, blocked by a suitable intervention, then a trial randomizing both exposure and the intervention can separate direct from indirect effects. Nonetheless, the estimation must be carried out using the G-computation algorithm. Conventional adjustment methods remain biased. When exposure and the intermediate interact to cause disease, direct and indirect effects will not be separable even in a trial in which both the exposure and the intervention blocking intermediate effects are randomly assigned. Nonetheless, in such a trial, one can still estimate the fraction of exposure-induced disease that could be prevented by control of the intermediate. Even in the absence of an intervention blocking the intermediate effect, the fraction of exposure-induced disease that could be prevented by control of the intermediate can be estimated with the G-computation algorithm if data are obtained on additional confounding variables.
Algorithms↗
Simplifying the evaluation of primary health care programmes.
Currently, there is considerable attention for health impact--as measured by mortality, morbidity or nutrition indicators, in the evaluation of primary health care (PHC) programmes. In most cases, health impact evaluations tend to be dominated by methodological discussions on data collection, analysis and interpretation, which are not relevant to the majority of PHC programmes. In this paper a theoretical framework of variables, affecting child survival, is presented. The key to this action-oriented framework is the identification of a set of intermediate variables which directly affect the health status of children, but can be influenced by PHC interventions as well. It is recommended that evaluations of PHC programmes should focus on these intermediate variables and be less concerned with health impact of the interventions.
Developing Countries↗
Fallibility in estimating direct effects.
We use causal graphs and a partly hypothetical example from the Physicians' Health Study to explain why a common standard method for quantifying direct effects (i.e. stratifying on the intermediate variable) may be flawed. Estimating direct effects without bias requires that two assumptions hold, namely the absence of unmeasured confounding for (1) exposure and outcome, and (2) the intermediate variable and outcome. Recommendations include collecting and incorporating potential confounders for the causal effect of the mediator on the outcome, as well as the causal effect of the exposure on the outcome, and clearly stating the additional assumption that there is no unmeasured confounding for the causal effect of the mediator on the outcome.
Aspirin↗
Path analysis on the risk of mortality in very low birth weight infants.
OBJECTIVE: The purpose of this work is to clarify by path analysis which of the predictor factors of mortality in a cohort of 423 singletons VLBW infants born at Beilinson Medical Center during the years 1980-1990 (pre-surfactant era) exert direct or indirect effect on mortality. A direct risk factor was defined as a factor having an immediate effect on the outcome without working through an intermediate variable. By contrast, an indirect risk factor was defined as a factor that does not have a direct effect on the risk of mortality but instead works through an intermediate variable. Evaluation of mortality was done at three points over time (pre-delivery, at birth and post-delivery). RESULTS: In the pre-delivery model, gestational age, appropriateness for gestational age, mother's age and fetal distress work directly on mortality in VLBW infants. At birth, mother's age, birth weight and Apgar score at the fifth minute work directly on the risk of mortality. When the assessment of the risk of death was done post-delivery, mother's age, birth weight, Apgar score at the fifth minute and seizures influence directly on mortality risk, and fetal distress, gestational age and pre-natal induced hypertension (PIH) operate indirectly on the risk of mortality. CONCLUSION: The methodology we have used can be adopted in other investigations to distinguish and measure the effect of prognostic factors on the risk of an outcome.
Causality↗
G-estimation of causal effects: isolated systolic hypertension and cardiovascular death in the Framingham Heart Study.
Time-dependent covariates are often both confounders and intermediate variables. In the presence of such covariates, standard approaches for adjustment for confounding are biased. The method of G-estimation allows for appropriate adjustment. Previous studies applying the G-estimation method have addressed effects on all-cause mortality rather than on specific causes of death. In the present study, a method to adjust for censoring by competing risks is presented. The authors used the approach to estimate the causal effect of isolated systolic hypertension on cardiovascular mortality in the Framingham Heart Study, with a 10-year follow-up using data from 1956 to 1970. Arterial rigidity is a major determinant of isolated systolic hypertension and may be a confounder of the relation between isolated systolic hypertension and cardiovascular death. Conversely, isolated systolic hypertension may by itself contribute to stiffening of the vessel wall, and arterial rigidity may therefore also be an intermediate variable in the causal pathway from isolated systolic hypertension to cardiovascular death. While controlling for arterial rigidity and other baseline and time-dependent covariates, isolated systolic hypertension decreased the time to cardiovascular death by 45% (95% confidence interval 3-69).
Adult↗
Estimates of body composition as intermediate outcome variables: are DEXA and BIA ready for prime time?
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Absorptiometry, Photon↗
Effects of inhalation of ethyl-ether on glycemia and on some variables of intermediate metabolism in rats.
The influence of a anesthetic, ethyl-ether, on arterial plasma levels of glucose, insulin and lipids was studied in starved Wistar rats. Ethyl-ether increased significantly (P < 0.05) glucose plasma levels, as a result not only of stress and of the release of catecholamines and glucocorticoids, but also of the decrease in the use of glucose by the tissues. Ethyl-ether did not change significantly the level of triglycerides, cholesterol and phospholipids. Insulin concentration was not increased, even when hyperglycemia was established. Ketonuria, acidosis and hypercapnia were increased. In these rats the administration of insulin produced a diminution in glycemia. The findings suggest that, under anesthesia with ether, the endocrine pancreas is incapable of recognizing glucose as a specific stimulus to promote the release of insulin.
Acidosis↗
[Estimates of effects of the intermediate fertility variables in Bongaarts model].
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Abortion, Induced↗
Analysis of birth intervals in India's Uttar Pradesh and Kerala States.
Life tables of birth intervals and median birth intervals in two Indian states, Uttar Pradesh and Kerala, were computed for several subgroups of the study population. Multivariate hazards modelling technique was used to examine the net effect of each of the variables studied. The results show a substantial effect of socioeconomic variables in child-spacing after controlling for the major intermediate variables.
Adolescent↗
A causal approach to the study of fertility and familism.
This paper tests, within the framework of LISREL, the causal structures of fertility using data from the 1973-74 Growth of Alberta Family Study (GAFS) of women aged 18-44 who are currently married or living common-law. Differential fertility among two groups of women classified by nativity also are examined. The women's background characteristics (e.g., age, religiosity, and education) are viewed as exogenous variables. The endogenous variables are familism and expected family size; familism is designated as an intermediate variable in the model, linking demographic and socioeconomic (including cultural) factors to fertility, The results indicate that familism acts as an important variable explaining fertility, particularly, among foreign-born women. The study confirms and extends earlier research findings that religiosity and education influence couples' fertility, the former positively and the latter negatively.
Adolescent↗
The internal structure of the phenomenology of auditory verbal hallucinations.
BACKGROUND: Auditory verbal hallucinations (AVH) do not have uniform pathological significance. They affect patients with different brain disorders, and vary along multiple phenomenological dimensions. Evidence indicates that some of the phenomenological variables have specific neural substrates. Therefore, a comprehensive characterization of the phenomenological variations of AVH and the interrelationship between these variables was undertaken. METHOD: Twenty phenomenological variables were identified; on each AVH had a binary value (present or absent). Information about 11 of these variables were obtained from 30 patients. Hierarchical cluster (HC) and multidimensional scaling (MDS) analyses were performed to investigate the hidden structure and dimensions of these variables. RESULTS: HC yielded two main clusters with further sub-clusters in each. The first cluster included hallucinations with low linguistic complexity, repetitive content, attributed to self, located in outer space, and associated with different kinds of control strategies. The second cluster included hallucinations with high linguistic complexity, systematized content, multiple voices, attributed to others, and located in inner space. In MDS, three dimensions were identified: linguistic complexity, self-other attribution, and inner-outer space location. CONCLUSION: The patterns of clustering and dimensional configuration of AVH characteristics were in accord with intuitive expectation and validated the patients' descriptions of their experiences. These findings could reflect aspects of the neural mechanisms of AVH. For example, the presence of neural specificity for each phenomenological variable, intermediate neural commonality for groups of variables, and a final common pathway for all subtypes of AVH. Another example is a differential level of language dysfunction according to the linguistic complexity of AVH.
Adult↗
Socio-economic determinants of achieved fertility in Egypt.
The authors investigate socioeconomic determinants of fertility in Egypt. The paper "begins with the intermediate variables and then moves to [a] wider range of social, demographic and environmental influences....The results have shown that age and age related indicators (years since first union and age at first union) are the best group of independent indicators to explain number of children...."
Africa↗
Fertility on the US-Mexico border.
Using Bongaarts' model, the relative importance of the proximate determinants of fertility is explored in five populations on the US-Mexico border. For the groups closest to natural fertility (the two Mexican groups), lactation, use of contraception, and marriage all were moderately important in terms of their direct effect on fertility. For the group with lowest fertility (Anglo-American), contraceptive use was an important factor inhibiting fertility; marriage was important but not nearly as important as contraceptive use. For the two US Mexican-American groups, contraceptive use was an important intermediate variable, not as important as for Anglo-Americans, but more important than it was for the two populations in Mexico. The proportion married was a moderately important factor for the Mexican-American groups. For these five populations the principal differences in fertility rates result from substantial differences in the use of effective contraception. Bongaarts' model proved very useful as an analytical framework in this study.
Adolescent↗