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Biomedical subjects

B Reiser

Publications and source records attributed to B Reiser.

10 recordsLinked to original sources
PubMed2000

Measuring the effectiveness of diagnostic markers in the presence of measurement error through the use of ROC curves.

The area under the receiver operating characteristic curve is frequently used to assess the effectiveness of diagnostic markers in distinguishing between diseased and healthy individuals. These markers are generally subject to measurement error. In this paper confidence intervals for the area under the curve are developed which take measurement error into account. These intervals depend on the availability of replicated observations for the subject. Both equal and unequal numbers of replicates per subject are considered. The practice of averaging over replicates and then ignoring measurement error is examined and found wanting.

Adolescent↗
PubMed1999

[Generalized ROC criteria in the evaluation of several tumor markers].

The main objective of this paper is to present a method of evaluating several tumor markers by using the generalized ROC criterion. This criterion finds the best linear combination of the tumor markers such that the area under the ROC curve is maximized. Confidence intervals for the generalized ROC criteria are also presented. This methodology is applied to 51 patients with advanced colorectal cancer for whom the ACE tumor markers were measured before and during chemotherapy treatment. Two populations were defined according to clinical response to chemotherapy. Each marker taken separately, whether on the raw scale or on the transformed scale, contained 0.5 in the confidence interval and was thus non significant. This was also true for both markers on the raw scale. However, the best linear combination on the logarithms of ACE before and at evaluation gave a significantly better area under the ROC curve. A weighted change in ACE measurements significantly distinguishes between responders and non responders in patients with advanced colorectal cancer. We propose that the methodology presented in this paper be used for the evaluation of several tumor markers.

Analysis of Variance↗
PubMed1998

Cognitive knowledge decline after Advanced Trauma Life Support courses.

OBJECTIVE: To assess the cognitive knowledge decline among graduates of the Advanced Trauma Life Support (ATLS) program in Israel, to compare the rate of decline between surgeons and nonsurgeons, and to recommend appropriate timing for refresher courses. METHODS: A prospective study based on multiple-choice question test results of 220 ATLS course graduates was conducted 3 to 60 months after course completion. These results were then compared with the examination results immediately after the course. A statistical model based on survival analysis was used to evaluate the decline pattern and extent and to compare the study groups. RESULTS: A significant decline of cognitive knowledge over time among ATLS graduates was demonstrated. This decline was significantly greater in the nonsurgical group. A critical point of 20% cognitive knowledge loss among 50% of the examined physicians was observed around the 180th week after completion of the course. CONCLUSION: Physicians taking the ATLS course lose a significant part of their acquired cognitive knowledge after 3.5 years. Surgeons retain their cognitive knowledge for longer periods of time. Based on the study results, the optimal timing for a refresher course is between 3 and 4 years after the initial ATLS course.

Adult↗
PubMed1997

The accumulated experience of the Israeli Advanced Trauma Life Support program.

BACKGROUND: Between January 1990 and May 1995 one faculty in Israel taught Advanced Trauma Life Support (ATLS) courses to 3,700 physicians. Two types of courses were given to three subpopulations. We studied the influence of demographic variables on students' achievements in the course and compared students' achievements as a function of their course type. STUDY DESIGN: This study was conducted as a concurrent longitudinal study. RESULTS: Achievements of 3,700 students were analyzed. The precourse grade, type of course, and their interaction were found to have a significant effect on the postcourse grades. Physicians practicing surgical subspecialties, in general, did better, as did students educated in English-speaking countries. Students who took part in the Combat Trauma Life Support (CTLS) course, which included the entire ATLS course and additional lectures and exercises, also ended with better scores. CONCLUSIONS: Physician's country of origin and clinical subspecialty have a significant effect on the cognitive achievement in the ATLS course provided in Israel. An expanded ATLS course (CTLS), to include additional military trauma topics as well as additional skill station training, can improve the results of the postcourse grades.

Adult↗
PubMed1997

Confidence intervals for the generalized ROC criterion.

Receiver operating characteristic (ROC) curves are frequently used to assess the usefulness of diagnostic markers. When several diagnostic markers are available, they can be combined by a best linear combination: that is, when the area under the ROC curve of this combination is maximized among all possible linear combinations. This maximal area is the generalized ROC criterion, which provides a measure of how effective the combination of the markers is. This criterion needs to be estimated from the data, and is usually evaluated against single markers. In the present paper, we provide confidence intervals for the generalized ROC criterion under the assumption of homogeneous covariance matrices, derive an approximation for the heterogeneous covariance matrices case, and evaluate the approximation via a simulation study. Finally, we present an illustrative example.

Biomarkers↗
PubMed1995

Dependent masking and system life data analysis: Bayesian inference for two-component systems.

Data from field operations of a system is often used to estimate the reliability of components. Under ideal circumstances, this system field data contains the time to failure along with information on the exact component responsible for the system failure. However, in many cases, the exact component causing the failure of the system cannot be identified, and is considered to be masked. Previously developed models for estimation of component reliability from masked system life data have been based upon the assumption that masking occurs independently of the true cause of system failure. In this paper we develop a Bayesian methodology for estimating component reliabilities from masked system life data when the probability of masking is dependent upon the true cause of system failure. The Bayesian approach is illustrated for the case of a two-component system of exponentially distributed components.

Bayes Theorem↗
PubMed1995

Dynamic treatment allocation adjusting for prognostic factors for more than two treatments.

Methods of sequential allocation of one of K treatments to patients, while controlling for important prognostic factors, are developed and compared. We focus on methods that are based on optimality theory (Begg and Iglewicz, 1980, Biometrics 36, 81-90; Atkinson, 1982, Biometrika 69, 61-67), the permuted block procedure (Zelen, 1974, Journal of Chronic Diseases 27, 365-375), and the compromise method (Faraggi and Reiser, 1991, Communication in Statistics, Simulation and Computation 20, 243-254). These methods are extended to the K treatments case and are evaluated in terms of efficiency and balance. It is shown that each method achieved the best results in the criterion it was designed to optimize, i.e., within stratum balance for the permuted block allocation and efficiency for the allocations that are based on optimality theory, but did not do well with other criteria. The compromise method, on the other hand, has good overall properties in terms of both balance and efficiency.

Biometry↗
PubMed1986

Alternative estimation procedures for Pr(X less than Y) in categorized data.

Consider two independent random variables X and Y. The functional R = Pr(X less than Y) [or gamma = Pr(X less than Y) - Pr(Y less than X)] is of practical importance in many situations, including clinical trials, genetics, and reliability. In this paper several approaches to estimation of gamma when X and Y are presented in discretized (categorical) form are analyzed and compared. Asymptotic formulas for the variances of the estimators are derived; use of the bootstrap to estimate variances is also discussed. Computer simulations indicate that the choice of the best estimator depends on the value of gamma, the underlying distribution, and the sparseness of the data. It is shown that the bootstrap provides a robust estimate of variance. Several examples are treated.

Analysis of Variance↗
PubMed1977

Vaginal temperature rhythms in sheep.

Six ovariectomized adult sheep were tethered in a temperature--and light--controlled room and given feed and water ad lib. Vaginal temperatures were recorded at 10m intervals for 12 or 17d. Hourly means were calculated and subjected to statistical anlayses by time--series procedures. Initial analysis revealed a positive linear trend in most animals suggesting a low frequency rhythm with a period greater than the 17d experimental period. All subsequent analyses were, therefore, conducted on data adjusted for a linear trend. The periodogram, correlogram and spectral density analyses revealed a predominant circadian rhythm in 5 of the 6 animals. Ultradian rhythms, with periods of 12 and 8h, were also present in some animals. The magnitude of the rhythmic change in vaginal temperature, as estimated from the amplitude of the circadian component, varied from 0.05 degrees to 0.16 degrees C. Calculation of the external acrophase indicated that the daily maximum temperature occurred between 0851 and 1727.

Animals↗