Biomedical subjects

Antonio Ciampi

Publications and source records attributed to Antonio Ciampi.

6 recordsLinked to original sources

PubMed2003

Effects of exposure measurement error when an exposure variable is constrained by a lower limit.

Epidemiologic studies routinely suffer from bias due to exposure measurement error. In this paper, the authors examine the effect of measurement error when the exposure variable of interest is constrained by a lower limit. This is an important consideration, since often in epidemiologic studies an exposure variable is constrained by a lower limit such as zero or a nonzero detection limit. In this paper, attenuation of exposure-disease associations is defined within the framework of a classical model of uncorrelated additive error. Then, the special case of nonlinearity due to the effect of a lower threshold is examined. A general model is developed to characterize the effect of random measurement error when there is a lower threshold for recorded values. Findings are illustrated under the assumption that the true exposure follows the lognormal and gamma distributions. The authors show that the direction and magnitude of bias in estimated exposure-response associations depends on the population distribution of the exposure, the magnitude of the recording threshold, the value assigned to below-threshold measurement results, and the variance in the measured exposure due to random measurement error.

Bias↗

PubMed2002

A new approach to training back-propagation artificial neural networks: empirical evaluation on ten data sets from clinical studies.

We present a new approach to training back-propagation artificial neural nets (BP-ANN) based on regularization and cross-validation and on initialization by a logistic regression (LR) model. The new approach is expected to produce a BP-ANN predictor at least as good as the LR-based one. We have applied the approach to ten data sets of biomedical interest and systematically compared BP-ANN and LR. In all data sets, taking deviance as criterion, the BP-ANN predictor outperforms the LR predictor used in the initialization, and in six cases the improvement is statistically significant. The other evaluation criteria used (C-index, MSE and error rate) yield variable results, but, on the whole, confirm that, in practical situations of clinical interest, proper training may significantly improve the predictive performance of a BP-ANN.

Biometry↗

PubMed2002

Prediction trees with soft nodes for binary outcomes.

Consider the problem of predicting the occurrence of an event, the onset of diabetes mellitus, say, from a vector of continuous and discrete predictors. We propose a new algorithm for the construction of a tree-structured predictor for the event of interest, which uses a new approach for dealing with continuous predictors. The novelty is that the tree uses splits for continuous variables. This means that at each node an individual goes to the right branch with a certain probability, function of a predictor. The predictor as well as the particular shape of the function is chosen from the data by the proposed algorithm. We evaluate its performance on several real data sets, in particular comparing it with a standard tree-growing algorithm. We also present an analysis of a well-known data set, the Pima Indian diabetes data set, to illustrate the application of the method in biostatistics.

Aged↗

PubMed2002

Determinants of discontinuation of new courses of antihypertensive medications.

Discontinuation of medication use constitutes a major barrier to adequate control of high blood pressure. We examined the effect of an array of potential predisposing, enabling and reinforcing factors on the discontinuation of newly prescribed antihypertensive medications. We conducted a prospective cohort study through a network of 173 pharmacies across Canada where were identified individuals newly prescribed an antihypertensive monotherapy. We interviewed participants by telephone four times to obtain information for a minimum duration of 18 months after entry into the cohort. We analyzed data using a multivariate proportional hazard model. Of 682 eligible participants, 43.3% had discontinued their initial medication at the end of the observation period. Individuals more likely to discontinue their initial medication were those who perceived side effects from this medication [Hazard Ratio (HR) = 1.91; 95% Confidence Interval (CI) 1.47-2.47). Individuals with medication insurance coverage were less likely to discontinue (HR = 0.74; 95% CI 0.55-0.99). Persistence with newly prescribed medications could be improved by selecting antihypertensive medications containing fewer side effects and by lifting economic barriers to drug treatment.

Adolescent↗

PubMed2002

Uses and limitations of statistical accounting for random error correlations, in the validation of dietary questionnaire assessments.

OBJECTIVE: To examine statistical models that account for correlation between random errors of different dietary assessment methods, in dietary validation studies. SETTING: In nutritional epidemiology, sub-studies on the accuracy of the dietary questionnaire measurements are used to correct for biases in relative risk estimates induced by dietary assessment errors. Generally, such validation studies are based on the comparison of questionnaire measurements (Q) with food consumption records or 24-hour diet recalls (R). In recent years, the statistical analysis of such studies has been formalized more in terms of statistical models. This made the need of crucial model assumptions more explicit. One key assumption is that random errors must be uncorrelated between measurements Q and R, as well as between replicate measurements R1 and R2 within the same individual. These assumptions may not hold in practice, however. Therefore, more complex statistical models have been proposed to validate measurements Q by simultaneous comparisons with measurements R plus a biomarker M, accounting for correlations between the random errors of Q and R. CONCLUSIONS: The more complex models accounting for random error correlations may work only for validation studies that include markers of diet based on physiological knowledge about the quantitative recovery, e.g. in urine, of specific elements such as nitrogen or potassium, or stable isotopes administered to the study subjects (e.g. the doubly labelled water method for assessment of energy expenditure). This type of marker, however, eliminates the problem of correlation of random errors between Q and R by simply taking the place of R, thus rendering complex statistical models unnecessary.

Diet↗

PubMed2002

What influences family physicians' cancer screening decisions when practice guidelines are unclear or conflicting?

OBJECTIVES: To determine (a) the respondents' perceptions of 4 unclear or conflicting cancer screening guidelines: prostate-specific antigen (PSA) for men over 50, mammography for women 40-49, colorectal screening by fecal occult blood testing (FOBT), and colonoscopy for patients over 40; and (b) the influence of various factors on the decision to order these tests. STUDY DESIGN: National Canadian mail survey of randomly selected family physicians. POPULATION: Family physicians in active practice (n=565) selected from rural and urban family medicine sites in 5 provinces representing the main regions in Canada. OUTCOME MEASURED: Agreement with guideline statements, and decision to order screening test in 6 clinical vignettes. RESULTS: Of 565 surveys mailed, 351 (62.1%) were returned. Most respondents agreed with the Canadian Task Force recommendations, and most believed that various guidelines for 3 of the 4 screens were conflicting (PSA 86.6%; mammography 67.5%; FOBT 62.4%). Patient anxiety about cancer, patient expectations of being tested, and a positive family history of cancer increased the odds that the 4 tests would be ordered. A good quality patient-MD relationship decreased the odds of ordering a mammogram. Screening decisions were also significantly influenced by the respondents' beliefs about whether screening was recommended and whether screening could cause more harm than good. A physician's sensitivity to his or her colleagues' practice influenced screening decisions regarding PSA and mammography. CONCLUSIONS: These results suggest a conceptual framework for understanding the determinants of screening behavior when guidelines are unclear or conflicting.

Adult↗