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At least 19 recordsLinked to original sources
PubMed1996

Fructose-1,6-bisphosphatase. Primary structure of the rabbit liver enzyme. 'Intermediate' variability of an oligomeric protein.

The primary structure of rabbit liver fructose-1,6-bisphosphatase was determined by peptide analysis of digests with different proteases. The results establish the primary structure, complete data bank entries, and show that this enzyme variant is indeed homologous with other liver fructose-1,6-bisphosphatases. Residue differences with the enzymes from other mammals are 9-15%, with those from plants and yeasts about 50%, and with those from characterized prokaryotes up to 70%, showing an enzyme variability intermediate between those of 'variable' and 'constant' oligomeric dehydrogenases. Structural relationships, conformations and catalytic mechanisms are consistent within the family of fructose-1,6-bisphosphatases, and the rabbit protein is a typical rather than an aberrant form of the enzyme.

Adenosine Monophosphate↗
PubMed1982

Intermediate variables and educational differentials in fertility in Korea and the Philippines.

This analysis compares the effects of contraceptive use and infant and fetal mortality on the pace of fertility in Korea and the Philippines and explores the mediating effects of these intermediate variables on educational differentials in childspacing. For both intervals initiated in a recent period before a sample survey, second, third and higher-order intervals are examined. Transitions within successive segments of interval exposure (qx values) are examined rather than cumulative transitions (1 - lx values). This methodological choice is substantively important because breastfeeding should primarily affect early segments of exposure and because it allows empirical examination of the timing of the effects of other variables such as contraceptive use. Further, this choice allows multivariate analysis within the structure of the life-table perspective. The results show substantial differences in patterns between Korea and the Philippines, indicate clearly the effect of each intermediate variable, and illustrate how educational differentials in fertility are affected by contraception and infant and fetal mortality.

Adult↗
PubMed1989

The control of confounding by intermediate variables.

In epidemiologic studies of the effect of an exposure on disease, the crude association of exposure with disease may fail to reflect a causal association due to confounding by one or more covariates. Most previous discussions of confounding in the epidemiologic literature have considered only point exposure studies, that is, studies that measure exposure and covariate status only once, at start of follow-up. In this paper we offer definitions of confounding suitable for longitudinal studies that obtain data on exposure, covariate, and vital status at several points in time. An important difference between longitudinal studies and point exposure studies is that, in longitudinal studies, a time-dependent covariate can be simultaneously a confounder and an intermediate variable on the causal pathway from exposure to disease. In this paper I propose an estimator, the extended standardized risk difference, that provides control for confounding by a covariate that is simultaneously a confounder and an intermediate variable.

Data Interpretation, Statistical↗
PubMed1994

Intermediate variables as determinants of adverse pregnancy outcome in high-risk inner-city populations.

A probability sample survey of high-risk inner-city women with a live birth in the last 3 years shows that maternal medical risks and health behaviors during pregnancy are important intermediate variables influencing preterm delivery and birthweight. Women who developed two or more medical risks had about three-and-a-half times the risk of preterm delivery and two-and-a-half times the risk of low birthweight compared to those without such risks. Women with prior fetal loss had twofold increase in the risk of preterm delivery and low birthweight. Unintended pregnancy resulted in one-and-a-half to twofold increase in preterm delivery and low birthweight, respectively. Inadequate gestational weight increased the risk of preterm delivery by about 50%. Smoking during pregnancy raised the risk of low birthweight slightly more than one-and-a-half times.

Adolescent↗
PubMedDate not supplied

Determinants of fertility decline in China, 1981: analysis of intermediate variables.

This study examines the proximate determinants of fertility in China by making use of the data collected by the One-per-Thousand Sample Fertility Survey of 1982. The results indicate that the most important inhibitor of potential fertility is deliberate control. Its contribution to fertility change has been far greater than all other proximate determinants. The marital structure of the population is also an important factor, while lactational infecundability and induced abortion are relatively unimportant. Comparative results by using data from the In-depth Fertility Survey conducted in Shanghai Municipality, Hebei and Shaanxi Provinces in April 1985 agree well in the ranking of the four intermediate factors. The findings point to successful family planning program and government population policies, which propelled the fertility transition to a substantial degree. Further research needs and policy implications of the results of the study are discussed.

Adolescent↗
PubMed1982

Association of marital status and years of schooling with perinatal outcome: the influence of pre-natal care as an intermediate variable.

The association between mother's education and perinatal mortality, and between marital status and proportion of preterm deliveries was analyzed using data from 20,000 women and newborns delivered at the Hospital Barros Luco-Trudeau in Santiago, Chile. A highly significant correlation was found, but after being controlled by pre-natal care, that association disappeared for those mothers with good pre-natal care, remaining only as a part of the association for women who did not attend the pre-natal clinics or did not follow minimal standards of care. The definition used for "good pre-natal care" was much less demanding than WHO recommendation. We required a minimum of only 5 visits, starting before the 5th month of the pregnancy and with blood pressure and body weight registered at each visit. Pre-natal assistance was provided mostly by registered midwives, with occasional consultation by physicians. The efficiency of a low cost health activity, such as pre-natal care, in improving infant health is held in contrast with the inefficiency of high cost technology when applied to developing countries' health problems.

Chile↗
PubMedDate not supplied

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↗
PubMed1996

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↗
PubMed1992

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↗
PubMed1989

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↗
PubMedDate not supplied

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↗
PubMed1993

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↗
PubMedDate not supplied

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↗
PubMed1989

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↗
PubMed1993

Confounding or intermediate effect? An appraisal of iatrogenic bias in perinatal AIDS research.

Many epidemiological reports present estimates of exposure effect adjusted for a host of variables thought to be risk factors for the disease. The term iatrogenic bias is proposed to denote bias introduced by the analyst when inappropriately controlling for variables as though they were confounders. The focus of this commentary is iatrogenic bias resulting from the control of variables intermediate in the causal pathway between exposure and disease. Perinatal epidemiological studies are particularly vulnerable, and iatrogenic bias will typically mask or diminish the true effect of the exposure. The problem is illustrated with three examples from the field of perinatal AIDS epidemiology. It is recommended that researchers carefully scrutinize each variable considered for adjustment and use path diagrams to sort out causal pathways. In some instances, it may be most appropriate to present both adjusted and unadjusted estimates of effect.

Acquired Immunodeficiency Syndrome↗