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Results for “Effect Modifier, Epidemiologic”

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PubMed2005

Evidence of bias in estimates of influenza vaccine effectiveness in seniors.

BACKGROUND: Numerous observational studies have reported that seniors who receive influenza vaccine are at substantially lower risk of death and hospitalization during the influenza season than unvaccinated seniors. These estimates could be influenced by differences in underlying health status between the vaccinated and unvaccinated groups. Since a protective effect of vaccination should be specific to influenza season, evaluation of non-influenza periods could indicate the possible contribution of bias to the estimates observed during influenza season. METHODS: We evaluated a cohort of 72,527 persons 65 years of age and older followed during an 8 year period and assessed the risk of death from any cause, or hospitalization for pneumonia or influenza, in relation to influenza vaccination, in periods before, during, and after influenza seasons. Secondary models adjusted for covariates defined primarily by diagnosis codes assigned to medical encounters. RESULTS: The relative risk of death for vaccinated persons compared with unvaccinated persons was 0.39 [95% confidence interval (95% CI), 0.33-0.47] before influenza season, 0.56 (0.52-0.61) during influenza season, and 0.74 (0.67-0.80) after influenza season. The relative risk of pneumonia hospitalization was 0.72 (0.59-0.89) before, 0.82 (0.75-0.89) during, and 0.95 (0.85-1.07) after influenza season. Adjustment for diagnosis code variables resulted in estimates that were further from the null, in all time periods. CONCLUSIONS: The reductions in risk before influenza season indicate preferential receipt of vaccine by relatively healthy seniors. Adjustment for diagnosis code variables did not control for this bias. In this study, the magnitude of the bias demonstrated by the associations before the influenza season was sufficient to account entirely for the associations observed during influenza season.

Aged↗
PubMed1991

Polycystic ovaries and the risk of breast cancer.

Data from a case-control study that was conducted between 1980 and 1982 were analyzed to investigate the possible association between polycystic ovaries and the risk of breast cancer. The multicenter, population-based study included in-home interviews with 4,730 women with breast cancer and 4,688 control women aged 20-54 years. The age-adjusted odds ratio for breast cancer among women with a self-reported history of physician-diagnosed polycystic ovaries was 0.52 (95% confidence interval 0.32-0.87). The inverse association was not an artifact of infertility, age at first birth, or surgical menopause. Because women with this syndrome have abnormal levels of certain endogenous hormones, the observation of a low risk of breast cancer in this group may provide new insights into hormonal influences on breast cancer.

Adult↗
PubMed1992

Selection of controls in case-control studies. I. Principles.

A synthesis of classical and recent thinking on the issues involved in selecting controls for case-control studies is presented in this and two companion papers (S. Wacholder et al. Am J Epidemiol 1992;135:1029-50). In this paper, a theoretical framework for selecting controls in case-control studies is developed. Three principles of comparability are described: 1) study base, that all comparisons be made within the study base; 2) deconfounding, that comparisons of the effects of the levels of exposure on disease risk not be distorted by the effects of other factors; and 3) comparable accuracy, that any errors in measurement of exposure be nondifferential between cases and controls. These principles, if adhered to in a study, can reduce selection, confounding, and information bias, respectively. The principles, however, are constrained by an additional efficiency principle regarding resources and time. Most problems and controversies in control selection reflect trade-offs among these four principles.

Case-Control Studies↗
PubMed1990

Use of the confidence interval function.

Graphics displaying all confidence intervals around a point estimate have been referred to as P-value functions and consonance intervals. We recommend use of the term confidence interval function (CI function) rather than P-value function. The CI function is useful because it simultaneously depicts point estimation, variability, and the relation of these two factors to the null value. The usefulness of the CI function in demonstrating the concepts of effect modification and confounding, in meta-analysis, and in the comparison of various confidence interval procedures is evaluated. Software packages that produce CI functions are described.

Confidence Intervals↗
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↗
PubMedDate not supplied

The relationship between function, self-perception, and spinal deformity: Implications for treatment of scoliosis in children with spina bifida.

The purpose of this study was to determine the relationship of spinal deformity with physical function and self-perception in children with spina bifida. Ninety-eight eligible children with scoliosis and spina bifida were identified; 80 of them (82%) consented to participate. Spinal deformity was measured in many ways, including scoliosis, coronal balance, and pelvic obliquity. Measures of physical function included the Sitting Balance Scale, Jebsen Hand Scale, Hoffer Ambulation Scale, the Spine Bifida Spine Questionnaire, and the Activities Scale for Kids (ASK). Self-perception was determined with Harter's Self-Perception Profile. No relationship was found between spinal deformity and overall physical function (ASK). Of all aspects of spinal deformity, only coronal imbalance was significantly related to only one aspect of physical function (ie, sitting imbalance). No aspect of spinal deformity was related to self-perception. In conclusion, surgeons should be clear in their indications for surgery and recognize that in the short term the potential benefit of surgery may be, at best, to improve only sitting balance.

Activities of Daily Living↗
PubMed2000

Parents' and teachers' ratings of problem behaviours in children: genetic and contrast effects.

We obtained ratings on the Conners' scales from teachers (CTRS-28) and parents (CPRS-48) for 61 monozygotic and 64 dizygotic twin pairs, aged between 7 and 11 years. Model-fitting analyses were carried out to estimate the extent of genetic and environmental influences on problem behaviours, and to explore possible contrast effects in ratings by parents and teachers. Confirming previous findings with other measures, there was evidence of moderate to strong genetic effects on a range of problem behaviours. Parents' ratings on the Anxiety, Impulsive-Hyperactive and Learning Problem sub-scales showed significant evidence of contrast effects. There was no evidence of such rater bias or competitive sibling interaction effects in ratings by teachers, or in parents' ratings on the Conduct Problem and Psychosomatic sub-scales.

Analysis of Variance↗
PubMed1998

Residential magnetic fields and childhood leukemia: a meta-analysis.

OBJECTIVES: This article uses meta-analysis methodology to examine the statistical consistency and importance of random variation among results of epidemiologic studies of residential magnetic field exposure and childhood leukemia. METHODS: A variety of meta-analytic statistical methods were applied to all available studies combined and on sub-groups of studies chosen by exposure characteristics. Sample sizes and fail-safe n's were calculated to determine the robustness of results and the potential role of publication bias. RESULTS: Most studies show elevated but not statistically significant odds ratios. Results for exposures assessed by wire codes, distance, and/or historically reconstructed fields are relatively consistent, homogeneous, and positive, while those for direct magnetic field measurements are consistent, homogeneous, and marginally protective. Several unpublished studies, or a single unpublished study with several hundred subjects, would be needed to nullify the observed data. CONCLUSIONS: The observed results identify a consistent risk that cannot be explained by random variation. The data supporting magnetic fields as the principal risk factor are suggestive but inconsistent. Additional studies using innovative designs that focus on highly exposed children offer the most hope of untangling this issue.

Bias↗
PubMed1991

Prolonged breastfeeding and malnutrition: confounding and effect modification in a Brazilian cohort study.

We examined the association between prolonged breastfeeding and anthropometric status in a population-based cohort study of 5,914 liveborns from the city of Pelotas in Southern Brazil. When children from all socioeconomic groups were studied, there was no important association between current breastfeeding and anthropometric status at age 12 months. Children who were still breastfed at age 20 months--and, to a lesser extent, at 43 months--presented with poorer anthropometric status than their nonbreastfed counterparts. We did not find the same pattern in all socioeconomic groups, however. Children from low-income families who were breastfed tended to present better anthropometric status than those who were not, whereas the reverse was observed for children of middle- and high-income families. After controlling for confounding variables, the nutritional advantage of breastfeeding among low-income families was no longer clear, while the superiority of nonbreastfed infants amongst middle- and high-income children persisted. These findings indicate that some of the controversy regarding the nutritional effects of prolonged breastfeeding may have been caused by confounding and effect modification. Any decisions on whether or not breastfeeding should be encouraged after the first year of life should take into account the characteristics of the population as well as the anti-infective and birth-spacing properties of breastfeeding.

Anthropometry↗
PubMedDate not supplied

Sources of variation in spirometric measurements. Identifying the signal and dealing with noise.

Variation in spirometric lung function measurements can be ascribed to technical and biological sources and is conveniently classified as within- or between-subject and within- or between-population. A necessary first step is to identify which sources of variation constitute "noise" and which "signal" in the various applications in occupational medicine. This chapter proposes strategies to enhance signal and deal with noise, and it delineates areas in which additional research to strengthen existing information would maximize the usefulness of spirometric measurements.

Artifacts↗
PubMedDate not supplied

Measurement of nonspecific bronchial responsiveness in epidemiologic studies: methacholine challenge testing in the field.

Methacholine challenge testing is the most commonly used method for detecting and quantifying nonspecific bronchial hyperresponsiveness and has been used primarily in the clinical investigation of occupational asthma. This chapter reviews the rationale, methodology, and interpretation of this test which are relevant to conducting methacholine challenge testing outside the clinic ("in the field"). It also reviews the issues surrounding the methodology of methacholine challenge testing in general and, in particular, its use in epidemiologic studies.

Asthma↗
PubMedDate not supplied

How to design an effective research study.

The previous paper in this series (March 13) examined three methodological issues: sample selection, treatment and/or intervention and observation and measurement. Here, aspects of research design and procedures will be considered. Research design is important in helping to ensure that the results can be correctly interpreted and that observed changes can be attributed to the planned intervention rather than to other intervening factors. The description of the procedure explains how the selected methods were combined and describes the practical details of how the study was carried out.

Confounding Factors, Epidemiologic↗
PubMed1998

A potential pitfall in control of covariates in epidemiologic studies.

Control of covariates is essential in nonexperimental epidemiologic studies. Important covariates, such as smoking or alcohol consumption, often are crudely categorized in epidemiologic analyses. In this paper, I illustrate by both hypothetical and empirical examples that control of crudely categorized covariates can yield strongly misleading results. In particular, I show that, under certain conditions, control for crudely classified covariates can even be worse than not controlling for such covariates at all. I conclude that covariate specification is an issue that requires much more care than it commonly receives in epidemiologic analyses.

Case-Control Studies↗