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PubMed2004

Side effects of generic competition?

This study examined the relationship between generic drug market shares and the number of reported side effects. Yearly time-series data for the number of reported side effects and information on market shares, prices, and quantities from 1972 to 1996 were used in this study. Poisson and negative binomial regression models were used in the statistical analysis. The results show that increased generic market share increases the number of reported side effects for all estimated models. When studying the relationship at the substance level, increasing generic market shares increases the number of side effects for 7 of the 15 substances. Generic substitution laws and measures to increase generic competition may thus have unintended consequences since these results show a positive relationship between generic market shares and reported side effects.

Drugs, Generic↗
PubMed2000

Additive isotonic regression models in epidemiology.

Stone's method for assessing disease risk around a point source through isotonic regression is routinely used in spatial epidemiology. It is useful in situations where the relationship of risk with exposure (distance being commonly used as a surrogate variable) is assumed monotonic but otherwise of unknown form. This paper extends this method to non-spatial epidemiology, where typically two or more risk factors are present. The methodology described is based on the additive isotonic model approach of Bacchetti; versions appropriate to count (Poisson) data and case-control (binomial) data are described. In both cases, adjustment for covariates is incorporated, and a Monte Carlo method of hypothesis testing and interval estimation is presented. The methodology is illustrated through a case-control example concerning the analysis of the possible effect of preconceptional external ionizing radiation doses on the sex ratio at birth among children of fathers working at the Sellafield nuclear installation, Cumbria, U.K.

Case-Control Studies↗
PubMed2002

Statistical approaches to establishing vaccine safety.

In a vaccine safety trial, the primary interest is to demonstrate that the vaccine is sufficiently safe, rejecting the null hypothesis that the relative risk of an adverse event attributable to the new vaccine is above a prespecified value, greater than one. We evaluate the exact probability of type I error of the likelihood score test, with sample size determined by normal approximation, by enumeration of the binomial outcomes in the rejection region and show that it exceeds the nominal level. In the case of rare adverse events, we recommend the Poisson approximation as an alternative and develop the corresponding conditional and unconditional tests. We give sample size and power calculations for these tests. We also propose optimal randomization strategies which either (i) minimize the total number of adverse cases or (ii) minimize the expected number of subjects when the vaccine is unsafe. We illustrate the proposed methods using a hypothetical vaccine safety study.

Humans↗
PubMed2003

A 6-year cohort study on relationship between functional fitness and impairment of ADL in community-dwelling older persons.

We measured functional fitness in older subjects and performed a follow-up survey for 6 years to clarify whether the level of functional fitness at a given point contributes to prediction of the subsequent occurrence of impairment of functions necessary for independent living. The longitudinal data were obtained for 391 persons aged 60 years or over, who were independently living in the community. Four items of functional fitness, i.e. the ability to perform standing/sitting movements, ability to perform traveling movements, ability to perform housekeeping movements, and ability to perform personal grooming activities, were assessed. The relationship between functional fitness and the risk of the occurrence of impairment of independence in daily living was evaluated using a binomial logistic regression model. In males, all the 4 items of functional fitness were significantly related to the risk of impairment of activities of daily living (ADL). In females, however, none of the items was significantly related to the risk of ADL impairment. In conclusion, functional fitness appears to be valid as a predictive parameter of future occurrence of ADL impairment in relatively healthy and independent elderly males. However, its validity in elderly females needs further evaluation.

Activities of Daily Living↗
PubMed1999

Effect of insurance on prescription drug use by ESRD beneficiaries.

In this article the author reviews the prescription drug coverage policy in the Medicare End Stage Renal Disease (ESRD) program and examines the relationship between secondary insurance status and the number of medications prescribed for dialysis patients who had Medicare as their primary payer. Negative binomial models were used to examine this relationship. Findings in this study indicate that the number of secondary payers has a significant impact on the number of prescription drugs received by Medicare ESRD patients. Further research is needed to determine whether Medicare beneficiaries without secondary insurance are obtaining fewer prescriptions than needed or if those with greater coverage are obtaining more than needed.

Adult↗
PubMed2003

Combining principal component and spectral analyses with the method of moments in studies of quantal transmission.

This chapter considers methods for measurements of postsynaptic responses and simple approaches to the estimation of parameters of quantal release in synapses of the central nervous system of vertebrates. The use of these methods is illustrated by the analysis of single-fibre and "minimal" monosynaptic postsynaptic potentials (PSPs) or currents (PSCs) recorded from neurons of the frog spinal cord and rat hippocampus. First, we briefly discuss traditional methods of the response measurements using peak amplitudes or areas, further focusing on a novel method based on multivariate statistical techniques of the principal component analysis (PCA). This approach provides typically better signal-to-noise ratios and is able to separate two or more response components, which can arise due to activation of more than one presynaptic fibre, axon collaterals, receptor subtypes or spatially separated transmission sites. Second, spectral analysis is introduced as the method of choice to verify whether the amplitude fluctuations of the postsynaptic responses have a quantal nature and to obtain estimations of the "basic" quantal parameters, i.e. the quantal size (Q) and mean quantal content (m), without introducing assumptions on release statistics. Third, we show how the method of moments could be applied in the framework of the Poisson and binomial models to estimate the basic quantal parameters and parameters p and n, which reflect the release probability and maximum number of quanta released (or the number of effective release sites), respectively. Fourth, we show that the analysis of the moments can also be instrumental to reveal non-uniformity of release probabilities and compare how several competing models of neurotransmitter release fit to multiple experimental data sets.

Animals↗
PubMed2003

Comparison of alpha/beta estimates from homogeneous (individual) and heterogeneous (population) tumor control models for early stage prostate cancer.

Radiobiological parameter estimates for prostate cancer are obtained from both a homogeneous (individual) and heterogeneous (population) tumor control model based on Poisson statistics and the linear quadratic model of cell survival. Parameter estimates for both models are highly correlated: statistically equivalent fits are achievable using either (1) linear quadratic (LQ) parameters with low numbers of radioresistant tumor stem cells, or (2) LQ parameters with corresponding larger number of radiosensitive tumor stem cells. A theoretical framework is developed to explain this correlation. A Monte Carlo error analysis based on binomial statistics is used to estimate confidence intervals for all parameter estimates. It was found that both the homogeneous and heterogeneous models produce approximately equivalent estimates of radiobiological parameters, including the alpha/beta ratio. However, the 95% confidence interval for the alpha/beta ratio derived from the heterogeneous model are considerably larger than those derived from the homogeneous model, which indicate the homogeneous model overestimates the statistical significance of the alpha/beta estimate.

Cell Survival↗
PubMed2005

A simple hybrid variance estimator for the Kaplan-Meier survival function.

In this paper, we propose a hybrid variance estimator for the Kaplan-Meier survival function. This new estimator approximates the true variance by a Binomial variance formula, where the proportion parameter is a piecewise non-increasing function of the Kaplan-Meier survival function and its upper bound, as described below. Also, the effective sample size equals the number of subjects not censored prior to that time. In addition, we consider an adjusted hybrid variance estimator that modifies the regular estimator for small sample sizes. We present a simulation study to compare the performance of the regular and adjusted hybrid variance estimators to the Greenwood and Peto variance estimators for small sample sizes. We show that on average these hybrid variance estimators give closer variance estimates to the true values than the traditional variance estimators, and hence confidence intervals constructed with these hybrid variance estimators have more nominal coverage rates. Indeed, the Greenwood and Peto variance estimators can substantially underestimate the true variance in the left and right tails of the survival distribution, even with moderately censored data. Finally, we illustrate the use of these hybrid and traditional variance estimators on a data set from a leukaemia clinical trial.

Analysis of Variance↗
PubMed2002

Medical service utilization by veterans seeking help for posttraumatic stress disorder.

OBJECTIVE: Posttraumatic stress disorder (PTSD) has been associated with higher rates of health complaints and medical conditions diagnosed by physicians, yet research examining the relationship between PTSD and health care utilization has been limited. This study compared the health service use of veterans with PTSD to that of help-seeking veterans without PTSD. The relationship between severity of PTSD and service utilization was also examined. METHOD: Data were collected from 996 veterans seeking an evaluation at a Veterans Affairs (VA) Medical Center specialty PTSD clinic in the southeastern United States between March 1992 and September 1998. Data included sociodemographic characteristics, severity of PTSD, and disability status. The outcome variable, VA health service utilization, was prospectively assessed 1 year from the date of the initial PTSD assessment. RESULTS: Although the use of VA mental health services by patients with PTSD was substantial (a median of seven clinic stops), these patients used more services in general physical health clinics that provided predominantly nonmental health services (a median of 18 clinic stops). Negative binomial regression models revealed that younger veterans with PTSD had greater health care utilization than those without PTSD who also sought services. Greater severity of PTSD was related to higher rates of mental and physical health service use among veterans without a service-connected disability. CONCLUSIONS: PTSD is associated with substantial health service use. The results highlight the importance of increased collaboration between primary care and mental health specialists, given that patients with PTSD are more likely to receive treatment in nonmental health clinics.

Age Distribution↗
PubMed1998

Analysis, assessment, and presentation of risk-adjusted statewide obstetrical care data: the StORQS II study in Washington State. Statewide Obstetrics Review and Quality System.

OBJECTIVE: To develop an effective, concise presentation of hospital-specific birth event and delivery-related complication rates, including significant deviations from expected rates calculated using risk-adjusted peer hospital data, for distribution to all Washington State hospitals with delivery services. DATA SOURCES: Complete data for calendar year 1993, including inpatient discharge records for mothers and newborns, birth certificates, and infant death records, for 74 hospitals from Washington state source files. STUDY DESIGN: Institutions were classified into four peer groups based on presence of neonatal intensive care units, number of births, and rural/urban location. Twenty-three clinical indicators of procedure (e.g., cesarean section) and complication rates were analyzed and presented. METHODS: For each indicator, observed and expected rates (adjusted within peer group for categorized baseline risk factors) were calculated and presented by institution. Effective graphic and numeric techniques for presenting significant deviations from expected rates were developed. Results were calculated in terms of numbers of events as well as rates. Approaches applicable to institutions with small numbers of deliveries were selected. PRINCIPAL FINDINGS: Exact confidence intervals (C.I.s) for event rates were superior to binomial or Poisson approximations for small hospitals. For calculating expected rates, indirect adjustment was used due to small numbers within risk factor categories. For all indicators, observed and expected rates along with 95 percent C.I.s for the true rate were presented graphically by institution for each peer group. Transforming C.I.s into "statistically acceptable ranges" allowed hospital personnel to assess their performance in terms of actual numbers of events as well as rates. CONCLUSIONS: Readily available statistical methods and straightforward descriptive approaches allow accurate presentation of outcomes for both large and small institutions.

Adult↗
PubMed1995

Influences on breast-feeding by lower-income women: an incentive-based, partner-supported educational program.

OBJECTIVE: To determine the effects of a partner-supported, incentive-based educational program on rates and duration of breast-feeding among low-income women. DESIGN: Women who expressed a willingness to participate in the breast-feeding educational program were randomly assigned to one of two groups: an intervention group and a control group who received usual breast-feeding education. SETTING: Clinics of the Special Supplemental Food Program for Women, Infants, and Children in Flagstaff, Ariz. SUBJECTS: Sixty-eight primiparous pregnant women with expected due dates between May 1992 and December 1992 were willing to participate in the study. Of these, 34 were randomly assigned to the intervention group and 34 to the control group. Approximately 81% of the women completed the study: 29 in the control group and 26 in the intervention group. INTERVENTION: The intervention consisted of special incentives (prizes) for women and their partners to participate in a breast-feeding class for expectant couples and an educational series on childbirth. Women were also encouraged to use a breast-feeding support program in which peers serve as role models. MAIN OUTCOME MEASURES: The primary outcome measure was infant feeding method. Data were collected from mothers in both groups at the time of discharge from the hospital and at 2 weeks, 6 weeks, and 3 months postpartum. STATISTICAL ANALYSES PERFORMED: Binomial proportional analyses of the feeding data were performed. RESULTS: Women in the intervention group reported a higher percentage of breast-feeding at all measurement times. APPLICATIONS: These findings suggest that incentives, such as donated prizes, can be used to attract primiparous women from lower socioeconomic groups, along with their partners, to participate in educational interventions designed to promote breast-feeding. Participation by couples in breast-feeding promotion activities can dramatically increase the rate and duration of breast-feeding.

Adolescent↗
PubMed1995

Ambient air pollution and hospitalization for congestive heart failure among elderly people in seven large US cities.

OBJECTIVES: Preexisting data sets were used to investigate the association between hospital admissions for congestive heart failure and air pollutants. METHODS: Medicare hospital admissions data, ambient air pollution monitoring data, and meteorological data were used to create daily values of hospital admissions for congestive heart failure, maximum hourly temperature, and maximum hourly levels of carbon monoxide, nitrogen dioxide, sulfur dioxide, and ozone. Data were compiled for each of seven cities (Chicago, Detroit, Houston, Los Angeles, Milwaukee, New York, and Philadelphia) for 1986 through 1989. Single-pollutant and multipollutant models with adjustments for temperature, seasonal effects, and weekly cycles were used in conducting negative binomial regression analyses. RESULTS: Ambient carbon monoxide levels were positively associated with hospital admissions for congestive heart failure in the single-pollutant and multipollutant models for each of the seven cities. The relative risk of hospital admission for congestive heart failure associated with an increase of 10 ppm in carbon monoxide ranged from 1.10 in New York to 1.37 in Los Angeles. CONCLUSIONS: Hospital admissions for congestive heart failure exhibited a consistent association with daily variations in ambient carbon monoxide. This association was independent of season, temperature, and other major gaseous pollutants.

Age Distribution↗
PubMed1999

[Annual school and menarche rhythms (vacation-study)].

BACKGROUND: It has been hypothesized that the yearly menarche rhythm could be caused by the seasonal variation of photoperiod and temperature or by the annual distribution of the scholar vacation and study periods. AIM: To test the hypothesis that the distribution of study vacation periods is a condition that modifies the annual menarche rhythm. SUBJECTS AND METHODS: Two thousand ninety four school girls from Chile, 2,356 girls from Madras, India, 3,454 girls from Medellin, Colombia and 2,627 girls from Debrecen, Hungary, were studied. They were asked about the month of their menarche. Vacation months were considered those with more than 6 days of leave from school. RESULTS: The seasonal hypothesis was refuted because there were contradictions with the expected antithetical behavior in both hemispheres, there was a significant heterogeneity of the yearly menarche among girls from the same region, the expected cline of the menarche frequency variance from equator to poles was not observed, finding an antithetical cline instead and there was a significant heterogeneity among months of the same season. On the other hand, months with vacation periods coincided significantly with peaks of menarche, while study months had lower proportion of menarche (total binomial probability < 10(-6)). Girls whose month of menarche was the same as their month of birth, did not agree completely with the vacation-study hypothesis as the rest of the sample. CONCLUSIONS: Vacation periods influenced menarche rhythm. However, these periods coincide with most cultural events and this strong association needs further study to be considered causal. It is not possible to assume school stress as the main explanatory variable. Ontogenetic factors such as birth imprinting also can influence the menarche rhythm, as shown in girls whose month of menarche coincided with their month of birth.

Adolescent↗
PubMed2002

Traffic deaths and superstition on Friday the 13th.

OBJECTIVE: This study compared deaths from traffic accidents on Friday the 13th with those on other Fridays in a national population. METHOD: The author examined the daily deaths from traffic accidents by sex and age and the mean daily temperature in Finland, 1971-1997. Adjusted risk ratios for death on Friday the 13th versus other Fridays were obtained by negative binomial regression. RESULTS: In men, the adjusted risk ratio for dying on Friday the 13th, compared with other Fridays, was 1.02, but for women, it was 1.63. An estimated 38% of traffic deaths involving women on this day were attributable to Friday the 13th itself. CONCLUSIONS: Friday the 13th may be a dangerous day for women, largely because of anxiety from superstition. The risk of traffic deaths on this date could be reduced by one-third, although the absolute gain would remain very small: only one death per 5 million person-days.

Accidents, Traffic↗
PubMedDate not supplied

A comparison of alternative models of prescription drug utilization.

The two-part estimation technique has been advocated for estimating models using individual level health care utilization data characterised by a large proportion of non-consumption and small proportions of heavy users. This paper compares the two-part model to several other estimators, including the Poisson, negative binomial and 'zero altered' negative binomial models on the basis of within-sample forecasting accuracy and non-nested model selection tests. The empirical model estimates the differential effect of the removal of copayments for prescription medicines on the prescription drug utilization by older adults with differing levels of health status. The two-part estimator of this model is found to dominate the competitors. Results from this model indicate that utilization increases appear to be higher among individuals with lower levels of health status.

Aged↗
PubMed2003

The relationship between neuropsychologic function and level of caregiver supervision at 1 year after traumatic brain injury.

OBJECTIVES: To evaluate distribution of levels of caregiver supervision at 1 year after traumatic brain injury, and to determine neuropsychologic predictors of supervision level. DESIGN: Prospective longitudinal design, concurrent measurement of neuropsychologic function and supervision level. SETTING: Seventeen Traumatic Brain Injury Model Systems centers. PARTICIPANTS: A total of 563 adults tested at 1 year postinjury; and a subgroup of 452 studied for neuropsychologic function in the absence of impairment in mobility or basic self-care, as assessed by high FIM instrument motor scores. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURE: Supervision level measured by scores on Supervision Rating Scale (SRS). RESULTS: Two thirds (69%) of the sample was rated as independent of supervision. Participants without significant dysfunction on motor FIM were grouped into supervision groups differing in intensity of time commitment from caregiver (independent, moderate supervision, heavy supervision). In univariate analyses, groups differed on demographic variables (education, race, productivity prior to injury), duration of altered consciousness, and all but 1 neuropsychologic measure. A binomial regression model (complementary log-log model) revealed that supervision at 1 year was predicted by education and scores on the Trail Making Test Part B and digits backward. CONCLUSIONS: Findings confirm the importance of preinjury status and measures of working memory and cognitive flexibility in predicting functional independence after TBI. The SRS appears prone to ceiling effects in persons followed prospectively after moderate to severe TBI.

Adolescent↗
PubMed2002

A Bayesian model for spatial wildlife disease prevalence data.

The analysis of the geographical distribution of disease on the scale of geographic areas such as administrative boundaries plays an important role in veterinary epidemiology. Prevalence estimates of wildlife population surveys are often based on regional count data generated by sampling animals shot by hunters. The observed disease rate per spatial unit is not an useful estimate of the underlying disease prevalence due to different sample sizes and spatial dependencies between neighbouring areas. Therefore, it is necessary to account for extra-sample variation and spatial correlations in the data to produce more accurate maps of disease incidence. The detection of spatial patterns is complicated by missing data in many of the geographical areas as the complete coverage of all areas is nearly impossible in wildlife surveys. For this purpose a hierarchical Bayesian model in which structured and unstructured over dispersion is modelled explicitly in terms of spatial and non-spatial components was implemented by Markov chain Monte Carlo methods. The model was empirically compared with the results of a non-spatial beta-binomial model using surveillance data of pseudorabies virus infections of European wild boars (Sus scrofa scrofa L.) in the Federal State of Brandenburg, Germany.

Animal Diseases↗
PubMed2006

Association between cannabinoid type-1 receptor polymorphism and body mass index in a southern Italian population.

CONTEXT: Endocannabinoids control food intake via both central and peripheral mechanisms, and cannabinoid type-1 receptor (CB1) modulates lipogenesis in primary adipocyte cell cultures and in animal models of obesity. OBJECTIVES: We aimed to evaluate, at the population level, the frequency of a genetic polymorphism of CB1 and to study its correlation with body mass index. DESIGN, SETTING AND PARTICIPANTS: Healthy subjects from a population survey carried out in southern Italy examined in 1992-1993 and older than 65 years (n=419, M=237, F=182) were divided into quintiles by body mass index (BMI). Two hundred and ten subjects were randomly sampled from the first, third and fifth quintile of BMI (BMI, respectively: 16.2-23.8=normal, 26.7-28.4=overweight, 31.6-49.7=obese) to reach a total of 70 per quintile. Their serum and white cells from the biological bank were used to measure the genotype and the blood variables for the study. MEASUREMENTS: Anthropometric parameters, blood pressure, serum glucose and lipid levels were measured with standard methods; genotyping for the CB1 1359G/A polymorphism was performed using multiplex PCR. Statistical methods included chi2 for trend, binomial and multinomial multiple logistic regression to model BMI on the genotype, controlling for potential confounders. RESULTS: We found a clear trend of increasing relative frequency of the CB1 wild-type genotype with the increase of BMI (P=0.03) and, using a multiple logistic regression model, wild-type genotype, female gender, age, glycaemia and triglycerides were directly associated with both overweight (third quintile of BMI) and obesity (fifth quintile of BMI). CONCLUSIONS: Although performed in a limited number of subjects, our results show that the presence of the CB1 polymorphic allele was significantly associated with a lower BMI.

Age Distribution↗