Biomedical subjects

J T Wassell

Publications and source records attributed to J T Wassell.

9 recordsLinked to original sources

PubMed1995

Frailty models of manufacturing effects.

The median service lifetime of respiratory safety devices produced by different manufacturers is determined using frailty models to account for unobserved differences in manufacturing processes and raw materials. The gamma and positive stable frailty distributions are used to obtain survival distribution estimates when the baseline hazard is assumed to be Weibull. Frailty distributions are compared using laboratory test data of the failure times for 104 respirator cartridges produced by 10 different manufacturers tested with three different challenge agents. Likelihood ratio tests indicate that both frailty models provide a significant improvement over a Weibull model assuming independence. Results are compared to fixed effects approaches for analysis of this data.

Equipment Failure↗

PubMed1994

Dose-response effects in an outbreak of Salmonella enteritidis.

The effects of ingested Salmonella enteritidis (SE) dose on incubation period and on the severity and duration of illness were estimated in a cohort of 169 persons who developed gastroenteritis after eating hollandaise sauce made from grade-A shell eggs. The cohort was divided into three groups based on self-reported dose of sauce ingested. As dose increased, median incubation period decreased (37 h in the low exposure group v. 21 h in the medium exposure group v. 17.5 h in the high exposure group, P = 0.006) and greater proportions reported body aches (71 v. 85 v. 94%, P = 0.0009) and vomiting (21 v. 56 v. 57%, P = 0.002). Among 118 case-persons who completed a follow-up questionnaire, increased dose was associated with increases in median weight loss in kilograms (3.2 v. 4.5 v. 5.0, P = 0.0001), maximum daily number of stools (12.5 v. 15.0 v. 20.0, P = 0.02), subjective rating of illness severity (P = 0.0007), and the number of days of confinement to bed (3.0 v. 6.5 v. 6.5, P = 0.04). In this outbreak, ingested dose was an important determinant of the incubation period, symptoms and severity of acute salmonellosis.

Adult↗

PubMed1994

Predictors of outcome in methadone programs: effect of HIV counseling and testing.

OBJECTIVE: To identify predictors of treatment outcomes in methadone maintenance programs and to determine whether HIV counseling and testing influenced these outcomes. DESIGN: Retrospective record review. SETTING: Four methadone maintenance programs in four cities in Connecticut, USA. PARTICIPANTS: Five hundred and ninety-four clients, who began treatment over an 18-month period and for whom records were available, took part. INTERVENTIONS: HIV counseling and testing. MAIN OUTCOME MEASURES: Risk of treatment discontinuation and persistent in-treatment illicit drug use. RESULTS: The most important predictor of treatment discontinuation and of persistent in-treatment illicit drug use was self-reported pretreatment cocaine use. After controlling for this and demographic risk factors, clients who received initial HIV counseling, when compared with clients who did not, had a similar 12-month discontinuation risk (54% vs 59%; P = 0.08) but were less likely to show persistent illicit drug use (46% vs 53%; P = 0.01). Among counseled entrants who were tested for HIV antibodies, those receiving positive results had a 12-month discontinuation risk similar to those receiving negative results (50% vs 52%), but more often showed persistent illicit drug use (57% vs 44%), although this difference may have been due to chance (P = 0.28). The majority of clients who discontinued treatment did so because they were discharged for noncompliance with clinic rules, usually for failing to pay fees. CONCLUSIONS: HIV counseling and testing do not have a substantial adverse effect on methadone treatment outcomes. In the clinics under study, failure to pay clinic fees was an important factor contributing to discontinuation of treatment.

AIDS Serodiagnosis↗

PubMed1993

A bivariate survival model with modified gamma frailty for assessing the impact of interventions.

Bivariate survival analysis models that incorporate random effects or 'frailty' provide a useful framework for determining the effectiveness of interventions. These models are based on the notion that two paired survival times are correlated because they share a common unobserved value of a random variate from a frailty distribution. In some applications, however, investigators may have some information that characterizes pairs and thus provides information about their frailty. Alternatively, there may be an interest in assessing whether the correlation within certain types of pairs is different from the correlation within other types of pairs. In this paper, we present a method to incorporate 'pair-wise' covariate information into the dependence parameter of the bivariate survival function. We provide an example using data from the Framingham Heart Study to investigate the times until the occurrence of two events within an individual: the first detection of hypertension and the first cardiovascular disease event. We model the dependence between these two events as a function of the age of the individual at the time of enrollment into the Framingham Study.

Adult↗

PubMed1993

Evaluation of protocol change in burn-care management using the Cox proportional hazards model with time-dependent covariates.

Survival analysis methods are valuable for detecting intervention effects because detailed information from patient records and sensitive outcome measures are used. The burn unit at a large university hospital replaced routine bathing with total body bathing using chlorhexidine gluconate for antimicrobial effect. A Cox proportional hazards model was used to analyse time from admission until either infection with Staphylococcus aureus or discharge for 155 patients, controlling for burn severity and two time-dependent covariates: days until first wound excision and days until first administration of prophylactic antibiotics. The risk of infection was 55 per cent higher in the historical control group, although not statistically significant. There was also some indication that early wound excision may be important as an infection-control measure for burn patients.

Anti-Bacterial Agents↗

PubMed1992

Predictors of outcome in methadone programs: effect of HIV counseling and testing.

OBJECTIVE: To identify predictors of treatment outcomes in methadone maintenance programs and to determine whether HIV counseling and testing influenced these outcomes. DESIGN: Retrospective record review. SETTING: Four methadone maintenance programs in four cities in Connecticut, USA. PARTICIPANTS: Five hundred and ninety-four clients, who began treatment over an 18-month period and for whom records were available, took part. INTERVENTIONS: HIV counseling and testing. MAIN OUTCOME MEASURES: Risk of treatment discontinuation and persistent in-treatment illicit drug use. RESULTS: The most important predictor of treatment discontinuation and of persistent in-treatment illicit drug use was self-reported pre-treatment cocaine use. After controlling for this and demographic risk factors, clients who received initial HIV counseling, when compared with clients who did not, had a similar 12-month discontinuation risk (54 versus 59%; P = 0.08) but were less likely to show persistent illicit drug use (46 versus 53%; P = 0.01). Among counseled entrants who were tested for HIV antibodies, those receiving positive results had a 12-month discontinuation risk similar to those receiving negative results (50 versus 52%), but more often showed persistent illicit drug use (57 versus 44%), although this difference may have been due to chance (P = 0.28). The majority of clients who discontinued treatment did so because they were discharged for non-compliance with clinic rules, usually for failing to pay fees. CONCLUSIONS: HIV counseling and testing do not have a substantial adverse effect on methadone treatment outcomes. In the clinics under study, failure to pay clinic fees was an important factor contributing to discontinuation of treatment.

Adult↗

PubMed1991

Surveillance of attempted suicide among adolescents in Oregon, 1988.

In January 1988, Oregon became the first state to require hospital-based reporting of attempted suicide (AS) in all adolescents less than 18 years old. From January to December 1988, 644 cases of AS were reported (annual rate of 214 per 100,000 population, ages 10 to 17 years). We compared these 644 cases of AS with all 137 Oregon adolescents less than 18 years old who committed suicide in Oregon during the 10-year-period 1979 through 1988, and found that the strongest predictor of outcome was method used.

Adolescent↗

PubMed1990

Risk factors for measles in a previously vaccinated population and cost-effectiveness of revaccination strategies.

Using data from a large measles outbreak that occurred in Dane County (Wisconsin) in 1986, we conducted a case-control study to evaluate risk factors for vaccine failure and assessed the cost-effectiveness of school-based revaccination strategies. Vaccination before a change in the measles vaccine stabilizer in 1979 (odds ratio, 5.5; 95% confidence interval, 1.05 to 28.9) and vaccination before age 15 months (odds ratio, 13.9; 95% confidence interval, 5.9 to 32.6) were identified as risk factors. Revaccination strategies for all students ($3444 per case prevented), students vaccinated before 1980 ($3166 per case prevented), and students vaccinated before age 15 months ($2546 per case prevented) were evaluated, assuming use of measles-mumps-rubella vaccine after the initial case was detected in a school. However, a large proportion of cases (43% to 53%) may not have been preventable using these strategies. Therefore, revaccination in all schools assessed to be at risk for measles may be necessary to prevent large outbreaks until a two-dose vaccination schedule is fully implemented.

Adolescent↗

PubMedDate not supplied

Power determination for geographically clustered data using generalized estimating equations.

Study designs in public health research often require the estimation of intervention effects that have been applied to a cluster of subjects in a common geographic area, rather than randomly assigned to individual subjects, and where the outcome is dichotomous. Statistical methods that account for the intracluster correlation of measurements must be used or the standard errors of regression coefficients will be under-estimated. Generalized estimating equations (GEE) can be used to account for this correlation, although there are no straightforward methods to determine sample-size requirements for adequate power. A simulation study was performed to calculate power in a GEE model for a proposed study of the effect of an intervention, designed to reduce lower-back injuries among nursing personnel employed in nursing homes. Nursing homes will be randomly assigned to either an intervention or control group and all employees within a nursing home will be treated alike. Historical injury data indicates that the baseline-injury risk for each home can be reasonably modelled using a beta distribution. It is assumed that the risk for any individual nurse within a nursing home follows a Bernoulli probability distribution expressed as a logit function of fixed covariates, which have values of odds ratios determined from previous studies which represent characteristics of the study population, and a random-intercept term which is specific for each home. Results indicate that failure to account for intracluster correlation can lead to overestimates of power as well as inflation of type I error by as much as 20 per cent. Although the GEE method accounted for the intracluster correlation when present, estimates of the intracluster correlation were negatively biased when no intracluster correlation was present. In addition, and possibly related to the negatively biased estimates of intracluster correlation, we also found inflated type I error estimates from the GEE method.

Algorithms↗