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J M Robins

Publications and source records attributed to J M Robins.

At least 55 records · Page 3Linked to original sources
PubMed1988

Conceptual problems in the definition and interpretation of attributable fractions.

We have argued that the concept of attributable fraction requires separation into the concepts of excess fraction, etiologic fraction, and incidence-density fraction. These quantities do not necessarily approximate one another, and the etiologic fraction is not generally estimable without strong biologic assumptions. For these reasons, care is needed in deciding which (if any) of the concepts is appropriate for a particular application. It appears that the excess fraction (like incidence proportion) will be most relevant in situations that require only consideration of whether disease occurs by a particular time. In situations that require consideration of when disease occurs, direct measures of effect on incidence time may be as relevant as or more relevant than any attributable fraction. To avoid technical complications, we have not discussed additional problems of causal attribution that can arise when exposure has multiple levels or is sustained over time, and the estimation problems that can arise when considering case-control studies, competing risks, or differential censoring. For more detailed discussions of such problems and proposed solutions, see references 11-20.

Epidemiology↗
PubMed1987

Analysis of proportionate mortality data using logistic regression models.

When only proportionate mortality data are available to an investigator studying the effect of an exposure on a particular cause of death, controls must be selected from among persons dying of other causes believed to be uninfluenced by the exposure under study. When qualitative or quantitative estimates of exposure history can be obtained for the deceased individuals, it is shown that one can use logistic regression models for the mortality odds to efficiently estimate the effect of exposure while controlling for relevant confounding factors by incorporating a priori information on baseline mortality rates available from US life tables. The proposed method is used to reanalyze data from a cohort of arsenic-exposed workers in a Montana copper smelter.

Adult↗
PubMed1987

Use of MR imaging in an outpatient MR center.

Indications for MR examinations and patient characteristics are evaluated for 4561 MR examinations performed at a freestanding outpatient MR imaging center between May 1984 and June 1986. Hospitalized patients accounted for less than 3% of the case load. Examinations of the head and spine accounted for 60% and 31% of the work load, respectively. Patients 65 years or older made up 15% of the case load during 1984 and 1985 and 21% in 1986. Referrals from neurologists, internists, and neurosurgeons accounted for 56%, 11% and 9% of patients, respectively. The percentage of patients who had CT, myelography, and other imaging procedures performed before referral for MR imaging declined significantly between 1984 and 1986. Indications for examination were mostly neoplastic diseases; degenerative diseases of the CNS, including multiple sclerosis; other disorders of the CNS; and disk diseases. Approximately 40% of all examinations were interpreted as normal. The number of patients referred for degenerative intervertebral disk disorders increased substantially between 1984 and 1985. This study documents the increasing acceptance of MR imaging as an important primary imaging technique for a variety of conditions, particularly those of the brain and spine.

Aged↗
PubMed1986

Estimation of ventilatory capacity in subjects with unacceptable lung function tests.

Based on pulmonary function data collected annually for six years on 540 Vermont granite workers, FEV1 in survey 1 was estimated by extrapolating back from subsequent measurements. The extrapolation method was found to fit the observed data of subjects with reproducible initial values very well (R2 = 0.87). Extrapolated FEV1s for workers unable to perform an adequate pulmonary function test according to the standards of the American Thoracic Society were compared to extrapolated values in the rest of the cohort. After adjusting for confounding, subjects with test failure in survey 1 had a lower extrapolated FEV1 than the rest of the cohort (p = 0.07). The mean extrapolated FEV1 of the 71 workers with an initial test failure was only 95% of a predicted value derived from the group with reproducible data, and the per cent predicted decreased from 98% to 71% as the number of test failures in the follow-up surveys increased (p = 0.0004). The American Thoracic Society and the Epidemiology Standardization Project currently recommend that test failures be excluded from the analysis of epidemiological data. Our findings suggest that alternative strategies for handling non-reproducible lung function may need to be explored in order to avoid selection bias.

Adult↗
PubMed1986

Identifiability, exchangeability, and epidemiological confounding.

Non-identifiability of parameters is a well-recognized problem in classical statistics, and Bayesian statisticians have long recognized the importance of exchangeability assumptions in making statistical inferences. A seemingly unrelated problem in epidemiology is that of confounding: bias in estimation of the effects of an exposure on disease risk, due to inherent differences in risk between exposed and unexposed individuals. Using a simple deterministic model for exposure effects, a logical connection is drawn between the concepts of identifiability, exchangeability, and confounding. This connection allows one to view the problem of confounding as arising from problems of identifiability, and reveals the exchangeability assumptions that are implicit in confounder control methods. It also provides further justification for confounder definitions based on comparability of exposure groups, as opposed to collapsibility-based definitions.

Bayes Theorem↗
PubMed1986

More on "Biased selection of controls for case-control analyses of cohort studies".

The authors consider several aspects of the design and analysis of synthetic case-control studies of cohort data under a proportional hazards model. First, in highly stratified data, consistent estimates of the relative risk are shown to result only if controls are sampled randomly with replacement from the entire risk set or without replacement from the noncases. Second, if previous controls are excluded from consideration as future controls but are included as cases if they fail, then inconsistent estimates of the relative risk can occur if "time" in the proportional hazards model represents an individual's chronological age and age at entry into follow-up is variable. On the other hand, if "time" represents time since the beginning of follow-up, estimates of the relative risk will be consistent, but the usual variance estimator will be inconsistent.

Biometry↗
PubMed1985

Confounding and misclassification.

The authors examine some recently proposed criteria for determining when to adjust for covariates related to misclassification, and show these criteria to be incorrect. In particular, they show that when misclassification is present, covariate control can sometimes increase net bias, even when the covariate would have been a confounder under perfect classification, and even if the covariate is a determinant of classification. Thus, bias due to misclassification cannot be adequately dealt with by the methods used for control of confounding. The examples presented also show that the "change-in-estimate" criterion for deciding whether to control a covariate can be systematically misleading when misclassification is present. These results demonstrate that it is necessary to consider the degree of misclassification when deciding whether to control a covariate.

Biometry↗
PubMed1985

Effects of spirometry standards in two occupational cohorts.

Characteristics of subjects with nonreproducible lung function tests (test failures) are described in two studies of occupational respiratory disease. According to current guidelines for spirometry, subjects with test failure are excluded from analyses of epidemiologic pulmonary function data. Among 415 Chinese cotton textile workers, the prevalence of byssinosis was 5.8% for subjects with repeatable tests and 13.3% for subjects with test failure. In regression analysis, the estimate of the association between cotton dust exposure and FEV1 decreased when subjects with test failure were excluded. In a second cohort of 378 asbestos-exposed machinists, the prevalence of chronic bronchitis was significantly greater among those with test failure. Considering only subjects with repeatable measurements, FEV1 was lower among textile workers with byssinosis and machinists with chronic bronchitis than among their asymptomatic coworkers. This suggests that, on average, subjects with poor test performance had lower (unobserved) FEV1 values, and therefore that the exclusion of subjects with test failure may cause selection bias.

Adult↗
PubMed1985

Soft tissue disorders in the upper limbs of female garment workers.

In this cross-sectional investigation of female garment workers the prevalence of soft tissue disorders of the hands and arms was studied. The findings were compared with the prevalence of disorders in a group of female hospital employees not required to use repetitive hand motion. One hundred and eighty-eight garment workers and 76 hospital employees were surveyed by questionnaire and physical examination. The prevalences of persistent shoulder, wrist, and hand pain were significantly greater among the garment workers (rate ratio 2, 4, and 3, respectively). In both groups about 60% of the persistent hand pain was consistent with carpal tunnel syndrome (rate ratio 3). These associations held when the comparisons were stratified by age and by length of employment. Workers whose native language was not English were significantly less likely to report symptoms (rate ratio 0.6). Workers in hand sewing and trimming suffered especially high prevalences of persistent pain in all upper limb sites. Stitchers had elevated rates of pain in the shoulders, wrists, and hands. Workers ironing by hand had a significant elevation in elbow pain rates. Garment assembly tasks appear to be associated with cumulative trauma of the hands and wrists; the biomechanical features of these jobs should be studied in greater detail.

Adult↗
PubMed1985

Estimation of a common effect parameter from sparse follow-up data.

Breslow (1981, Biometrika 68, 73-84) has shown that the Mantel-Haenszel odds ratio is a consistent estimator of a common odds ratio in sparse stratifications. For cohort studies, however, estimation of a common risk ratio or risk difference can be of greater interest. Under a binomial sparse-data model, the Mantel-Haenszel risk ratio and risk difference estimators are consistent in sparse stratifications, while the maximum likelihood and weighted least squares estimators are biased. Under Poisson sparse-data models, the Mantel-Haenszel and maximum likelihood rate ratio estimators have equal asymptotic variances under the null hypothesis and are consistent, while the weighted least squares estimators are again biased; similarly, of the common rate difference estimators the weighted least squares estimators are biased, while the estimator employing "Mantel-Haenszel" weights is consistent in sparse data. Variance estimators that are consistent in both sparse data and large strata can be derived for all the Mantel-Haenszel estimators.

Analysis of Variance↗
PubMed1984

Selection effects of repeatability criteria applied to lung spirometry.

The potential for introducing bias in studies of pulmonary function by the exclusion of subjects with nonrepeatable measurements was examined in a cohort of Vermont granite workers followed for five years. At each annual survey, a "test failure" was defined as a test in which the two largest forced expiratory volumes in one second (FEV1) differed by more than 200 ml. "Persistent test failure" was defined in terms of 1) the number of test failures for each worker over the six surveys and 2) the difference between the two best efforts at each survey, averaged over all surveys for each worker. The rate of FEV1 loss was estimated for each subject based only on repeatable measurements. It is widespread practice to exclude subjects from analysis who do not perform repeatable lung function tests. The authors found that subjects with persistent test failure were losing FEV1 at a faster rate than subjects without. The results suggest that the application of rigid repeatability criteria may bias epidemiologic findings by the exclusion of many subjects with accelerated loss of lung function.

Adult↗
PubMed1983

Depressed thyroid indexes associated with occupational exposure to inorganic lead.

The finding of low values for serum thyroxine and estimated free thyroxine in seven of 12 workers referred because of elevated blood lead levels (greater than 40 mg/L) prompted further investigation. In a cross-sectional study of workers at a small foundry, both measurements were found to regress negatively with blood lead level. In 12 of 47 subjects, both indexes were in the hypothyroid range. Serum thyrotropin and triiodothyronine levels in patients and study subjects with low indexes were all normal. Physical examinations failed to demonstrate the classic features of hypothyroidism. These data are compatible with a central depression of the thyroid axis or an alteration in thyroxine metabolism or binding to proteins. Irrespective of mechanism, the association between low thyroid indexes and elevated lead levels merits attention because of the large number of workers exposed to lead and the similarities between the clinical features of adult lead poisoning and hypothyroidism.

Adult↗
PubMed1981

Lipoid pneumonia caused by oil mist exposure from a steel rolling tandem mill.

Five of nine active tandem mill operators exposed at work to aerosolized hydrocarbon mist were referred for evaluation of respiratory complaints. The worker with the longest exposure had reduced lung volumes; he was admitted to the hospital for detailed study. Exercise studies revealed work load limited by ventilation and arterial oxygen desaturation. Flexible fiberoptic bronchoscopy with bronchoalveolar lavage and transbronchial biopsy revealed evidence of lipoid pneumonia. Assessment of the mill revealed levels of respirable oil mist by personal samplers throughout the area far below the currently accepted standard of 5 mg/M3. These findings confirm a 20-year-old hypothesis of J.G. Jones regarding the hazard of oil mist in this industrial setting.

Adult↗