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Biomedical subjects

R Tibshirani

Publications and source records attributed to R Tibshirani.

32 records · Page 2Linked to original sources
PubMed1994

Implications of measurement error in exposure for the sample sizes of case-control studies.

In this paper, recent results describing the effects of measurement error on estimation of the association between an exposure and a disease are applied to sample size calculation in case-control studies. Models of the relation between true exposure and a surrogate exposure measure assessed with error are used to derive equations for sample size determination. The results show that the sample size of a study based on an exposure variable which is measured with error must be larger by a factor of 1/rho 2 than if exposure were measured without error, where rho is the correlation between the true exposure and the surrogate exposure measure. Review of the magnitude of measurement error in dietary assessments suggests that failure to account for measurement error in sample size determination for case-control studies of diet and disease could lead to marked underestimation of the required sample size.

Bias↗
PubMed1994

Spinal deformity after multiple-level cervical laminectomy in children.

Considerable controversy exists in the orthopedic and neurosurgical literature over the true incidence and nature of spinal deformity after multiple-level cervical laminectomy in children. Eighty-nine patients with a mean radiographic follow up of 5.1 years (range 2-9 years) were reviewed. Mean age at surgery was 5.7 years (range 1 month-18 years). Most common diagnoses were Arnold-Chiari malformation, syringomyelia, or both (81%). Significant deformity developed in 46 patients (53%), with 33 developing a mean kyphosis of 30 degrees (range 5-105 degrees) and 13 developing a mean hyperlordosis of 62 degrees (range 40-95 degrees). Peak age at surgery of 10.5 years correlated weakly (P = 0.08) with the development of kyphosis. The development of hyperlordosis was strongly correlated (P = 0.01) with a peak age at surgery of 4.2 years. There was no correlation between diagnosis, sex, location, or number of levels decompressed and the subsequent development of deformity.

Adolescent↗
PubMed1993

Spatially localized neuronal cell lineages in the developing mammalian forebrain.

The role of cell lineage in the organization of the cerebral cortex and striatum of the developing rat forebrain was analysed using retroviral-mediated gene transfer to mark the progeny of individual progenitors. Injections around the onset of neurogenesis (embryonic day 14) produced neuronal- and glial-specific clones in the striatum and cortex. The majority of the neuronal clones were restricted to either the deep or superficial layers of the cortex and to either the striatal patch or matrix compartments of the striatum. Moreover, modeling the distributions of the neuronal clones in various ways revealed that grouping the clones into deep vs superficial cortical compartments and patch vs matrix striatal compartments best accounted for the clone distributions. These results suggest that at the onset of neurogenesis there is a heterogeneity of neuronal progenitors within the proliferative ventricular zone.

Animals↗
PubMed1993

Complications of use of the Ilizarov technique in the correction of limb deformities in children.

We reviewed the records of the first forty patients who had been managed at our institution with the Ilizarov technique for the correction of limb deformities, including limb-length inequality, to delineate the complications of this method of treatment. The duration of follow-up ranged from two and one-half to four years. A complication was defined as any untoward occurrence to a patient either during the course of treatment or after removal of the fixator. A major complication was considered one that necessitated an additional operative procedure; caused lasting sequelae, such as malunion, deformation of new bone, joint contracture or stiffness, or nerve palsy; or prolonged the treatment. A minor complication was regarded as one that responded to non-operative treatment and did not cause lasting sequelae, such as transient decreased motion of the joint, paresthesia, or pin-track infection. There were eighty-eight complications--thirty-eight, major, and the remaining fifty, minor--in the sixty-one segments of the limb that were treated; this represented an average of almost one and one-half complications for each segment. Twenty-nine unplanned operative procedures were performed either during treatment with the Ilizarov technique or after removal of the fixator. As anticipated, the prevalence of major complications was highest in the patients who had had more complex and prolonged treatment. Such complications were encountered less often as the surgeons gained experience with the procedure, but the rate of minor complications remained relatively constant, despite the increased experience.(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗
PubMed1992

Flexible covariate effects in the proportional hazards model.

The proportional hazards model is frequently used in analyzing the results of clinical trials, when it is often the case that the outcomes are right-censored. This model allows one to measure treatment effects and simultaneously identify and adjust for prognostic factors that might influence the outcome. In this paper, we outline a class of semiparametric models that allows one to model prognostic factors nonlinearly, and have the data suggest the form of their effect. The methods are illustrated in an analysis of data from a breast cancer clinical trial.

Adult↗
PubMed1992

Recent results of elective open cholecystectomy in a North American and a European center. Comparison of complications and risk factors.

Results of elective open cholecystectomy in 1252 patients treated in a North American and a European center were examined using a recent standardized classification of complications. Although there were significant differences between centers in population age, rate of concomitant disorders, and numbers of operators, the frequency and severity of complications were comparable. There were no deaths, but 12% and 14% of the patients developed complications in the two centers. About 6% of the patients developed grade I complications. Grade II complications were noted in 6% and 8%, and grade III in 0% and 0.3%. Using univariate and multivariate analysis, individual risk factors for developing complications were found to be different in the two centers. Two preoperative scoring systems, ASA and a simplified APACHE II, were predictive for complications in both centers, but did not account for all risk in these patients. Data from the two centers could not be combined because of significant interaction between risk factors and center. Elective open cholecystectomy is a safe procedure, particularly in terms of highly morbid complications and death. Generalization of risk factors identified in a particular center may be misleading because local conditions may significantly affect risk factors for complications. The data also demonstrate the advantages of a uniform way of reporting surgical complications, which may permit meaningful comparisons among centers.

Adolescent↗
PubMed1991

Clinical-physiologic correlations in acute asthma of childhood.

Seventy-one patients who presented to the emergency room with acute asthma were evaluated to determine the relationship between common clinical signs and spirometric and transcutaneous arterial oxygen saturation (SaO2) measurements. Prior to treatment, a physical examination was performed, a clinical score assigned, and pulmonary function and SaO2 were measured. Although forced expiratory volume in 1 second (FEV1) and SaO2 had strong correlation with the overall clinical score (r2 = .47, .49 respectively), many patients with low clinical scores and apparent mild clinical disease had low FEV1 values (as low as 20% predicted). Of the individual components of the clinical score (ie, heart rate, respiratory rate, pulsus paradoxus, accessory muscle use, dyspnea, and wheezing), the degree of accessory muscle use correlated most closely with lung function followed by the degree of dyspnea and wheezing. Similarly, the degree of accessory muscle use correlated most closely with SaO2 followed by dyspnea and respiratory rate. Significant correlation (r2 = .59) was found between SaO2 and FEV1, although the range of SaO2 value for a given FEV1 was wide and some patients with low FEV1 values had normal SaO2 values. These results show that although clinically apparent severe disease and hypoxemia were always associated with low FEV1, their absence does not exclude the presence of airflow obstruction. It is concluded that for the optimal evaluation of acute asthma in children in the emergency room, clinical evaluation should be used in conjunction with objective laboratory measurements.

Acute Disease↗
PubMed1990

Predicting the need for hospitalization in children with acute asthma.

In an attempt to identify factors which influence the decision of physicians to admit patients with acute asthma to the hospital, we studied prospectively 200 children (age 5.6 +/- 3.1 years, mean +/- SD) presenting to our emergency room with acute asthma. The children were assessed on arrival, and on disposition from the Emergency Room by one of the investigators. After obtaining historic data, a clinical score was assigned, and oxygen saturation and pulmonary function were measured. Of the 134 (67 percent) children who were discharged home from the Emergency Room, five returned within seven days and one was subsequently admitted. The clinical score on disposition was the sole variable found to best predict the decision for hospitalization (sensitivity 73 percent, specificity 95 percent). Of the variables obtained at presentation, the resulting decision tree found the clinical score to predict the decision for hospitalization (sensitivity 79 percent, specificity 75 percent). When the individual components of the clinical score were analyzed, the degree of dyspnea, as assessed by the investigator, was chosen as the rule to predict the hospitalization decision (sensitivity 88 percent, specificity 71 percent). We conclude that the decision with respect to the need for hospitalization in acute childhood asthma, is in practice based mainly on careful clinical evaluation. Pulmonary function and SaO2 measurements, although helpful adjuncts in the assessment of acute asthma, do not appear to contribute to the identification of patients who need hospital admission.

Asthma↗
PubMed1990

Exploring the nature of covariate effects in the proportional hazards model.

We discuss an exploratory technique for investigating the nature of covariate effects in Cox's proportional hazards model. This technique features an additive term sigma p1 fj(chi ij), in place of the usual linear term sigma p1 chi ij beta j, where chi i1, chi i2,...,chi ip are covariate values for the ith individual. The fj(.) are unspecified smooth functions that are estimated using scatterplot smoothers. These functions can be used for descriptive purposes or to suggest transformations of the covariates. The estimation technique is a variation of the local scoring algorithm for generalized additive models (Hastie and Tibshirani, 1986, Statistical Science 1, 297-318).

Algorithms↗
PubMed1987

Short-term economic change and the utilization of mental health facilities in a metropolitan area.

The relationship between economic change and mental disorder has been examined by several investigators over the past century. The purpose of this paper is to explore this basic relationship and determine its direction in a large Metropolitan area. A time series analysis was undertaken to investigate whether or not unemployment and indicators of mental health are related over time. Trends in the following four indicators of mental health were examined: 1. Number of admissions to all psychiatric facilities serving Metropolitan Toronto. 2. Number of admissions to one provincial psychiatric hospital (Queen Street Mental Health Centre) in Toronto, which serves a chronic population. 3. Number of discharges from Queen Street Mental Health Centre. 4. Number of admissions plus those assessed and not admitted to Queen Street Mental Health Centre. In order to assess the possible delayed effects of unemployment, correlation analyses were computed for several lag times. "Lag time" is defined here as the time delay between unemployment and its potential effects on mental health indicators. Lag times used were zero, three, six and twelve months. The best equation found was for a six month lag, indicating an inverse relationship; as unemployment increases, admissions and discharges decrease. Results suggest that there are no simple relationships between the dependent and independent variables. Observed trends may be due to much wider exogenous factors such as hospital capacity and changing admission criteria.

Community Mental Health Centers↗
PubMed1984

Describing health states. Methodologic issues in obtaining values for health states.

In health status index construction quantitative values for different states of health are frequently obtained by presenting written descriptions to raters whose values are elicited using one or more methods. In this study the authors examined the influence of several aspects of this measurement process upon the quantitative results obtained. They prepared a set of written descriptions of health states, each state being described in both a standard point-form and a narrative format. The narrative format was written in the first person singular, and listed all symptoms or problems associated with the state, whereas the point-form description included only the most severe symptom or problem. Values for these states were elicited from a group of 64 patients using two commonly employed methods, the standard gamble of Von Neumann and Morgenstern and category rating. The results indicate that the type of scenario presented to the rater and the sequence in which the methods of assessment were used had a major influence on the results. This work indicates that there is a need to examine systematically the process of obtaining quantitative values before reliance can be placed upon the results.

Adult↗
PubMed1982

The measurement of patients' values in medicine.

The standard gamble, derived from decision theory, is considered a criterion method for obtaining patients' values for different health states. In this method, a rater's value for a health state is obtained by determining, in a hypothetical setting, his readiness to remain in that state or take a risky choice with different outcomes of known value. According to the expected utility criterion, a rater's utility or value for a particular state should not be influenced by changes in the gamble outcomes. The standard gamble was used to obtain 64 patients' judgments of a set of health states, first with the outcomes of perfect health or death, and subsequently with other health states as outcomes. The changes in gamble outcomes significantly influenced reported values for health states, indicating that the standard gamble is internally inconsistent. This observation poses a major limitation to its use as a value measurement method in medicine.

Adult↗