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PubMed2005

Parametric modelling of cost data: some simulation evidence.

Recently, commentators have suggested that the distributional form of cost data should be explicitly modelled to gain efficiency in estimating the population mean. We perform a series of simulation experiments to evaluate the usual sample mean and the mean estimator of a lognormal distribution, in the context of both theoretical distributions and three large empirical datasets. The sample mean is always unbiased, but is somewhat less efficient when the population distribution is truly lognormal. However the lognormal estimator can perform appallingly when the true distribution is not lognormal. In practical situations, where the true distribution is unknown, the sample mean generally remains the estimator of choice, especially when limited sample size prohibits detailed modelling of the cost data distribution.

Clinical Trials as Topic↗
PubMedDate not supplied

Evaluation of the quality of clinical research studies of magnetic resonance angiography: 1991-1994.

To evaluate the quality of clinical research studies of magnetic resonance angiography (MRA), the authors reviewed studies of carotid, renal, and lower extremity MRA published in English during 1991-1994. The purpose, design, and implementation of the study, and the type of statistical information presented, were evaluated. The results were compared with those of previous studies that assessed the quality of MRI research. Fifty-nine studies of renal (n = 10), carotid (n = 29), and lower extremity (n = 20) MRA were identified; 30 met the inclusion criteria for the analysis. In these papers, presentation of a reference standard was routine, observer blinding was usual, information often was given about patient selection and exclusion, sample sizes were always given, and parameter estimates and statistical analyses were common. However, evidence of research planning was not routine, measurement of interobserver variability was occasional, variability of parameter estimates was usually not estimated, and validation of statistical assumptions was uncommon. On balance the authors conclude that the scientific quality of recent investigations of MRA is better than that of older studies of MRI.

Arteries↗
PubMed1995

A meta-analysis of bibliotherapy studies.

Used meta-analysis to examine the efficacy of bibliotherapy. Bibliotherapy treatments were compared to control groups and therapist-administered treatments. The mean estimated effect size (d) of the 70 samples analyzed was + 0.565. There was no significant differences between the effects of bibliotherapy and therapist-administered treatments, as well as no significant erosion of effect sizes at follow-up. Bibliotherapy did appear more effective for certain problem types (assertion training, anxiety, and sexual dysfunction) than for others (weight loss, impulse control, and studying problems). Overall the amount of therapist contact during bibliotherapy did not seem to relate to effectiveness, but there was evidence that certain problem types (weight loss and anxiety reduction) responded better with increased therapist contact. Recommendations for future research were given, especially for more research on the commonly purchased books and moderator analyses by personality type and reading ability.

Bibliotherapy↗
PubMed1993

Competence reasoning--handling ambiguous and imprecise data in the Pro.M.D. expert system shell.

The Pro.M.D. expert system shell (Pohl and Trendelenburg, Methods Inf Med 1988;27:111-117) is a computer program tool for knowledge-based computer interpretation of laboratory data. Competence reasoning deals with more or less vague knowledge. Vague knowledge is knowledge using vague data. In Pro.M.D. terminology, vague data are called 'semi-known'. Examples of semi-known numbers are numeric intervals like [5.2,8.8] (= all numbers between lower limit 5.2 and upper limit 8.8 are possible), or imprecise numbers like N[4,1] (= numbers vary around mean of 4 with standard deviation of 1). Semi-known qualitative data are ambiguous information on alternative values like low or normal (= from three alternatives low or normal are possible and high is impossible) or probability distributions on the alternatives like low in 60% or normal in 30% or high in 10% (= low, normal and high appear with the respective probabilities). Regarding vagueness, data may be divided into known data (unambiguous and precise information), semi-known data (ambiguous or imprecise information) or unknown data (no information). Semi-known data may be further subdivided into probabilistic data, where the probability distribution of domain values is known (e.g. Gaussian distribution) and possibilistic data, where probability distribution of domain values is not known (e.g. intervals). Probabilistic data may be obtained either by completely counting the population's individuals or by estimation from samples. In the latter case sample size is another measure for data vagueness. In the Pro.M.D. expert system shell functions are being implemented for input, output, calculation and testing of semi-known data. Thus, instruments for competence reasoning in Pro.M.D. will help to build up reliable and user friendly knowledge-based systems.

Clinical Laboratory Information Systems↗
PubMed1975

A method for prediciting localised areas of cerebral ischemia from visual evoked responses.

Responses to light flashes recorded from several scalp electrodes of a normal subject at rest and during alternate compression of the carotid arteries in the neck are analysed. The diffences found in response characteristics between the subject at rest and during carotid compression may serve as indicators of localised areas of cerebral ischemia. The results indicate a strong evidence that information about localised areas of cerebral ischemia is contained in the cepstra of the averaged visual evoked responses. The probability with which such areas can be localised using cepstral values as samples and linear multiple regression techniques for classification purposes has been estimated for our procedure and our sample size as being: Pa equals 79 plus or minus 17 per cent for left-right localisation, and Pb equals 61 plus or minus 16 per cent for left-none-right localisation.

Adult↗
PubMed1993

Design of the Lung Health Study: a randomized clinical trial of early intervention for chronic obstructive pulmonary disease.

The Lung Health Study is a multicenter randomized clinical trial. Participants are smokers aged 35-60 with spirometric evidence of moderate lung function impairment. The objective of the trial is to determine whether a program of smoking intervention and use of an inhaled bronchodilator can slow the rate of decline in pulmonary function over a 5-year follow-up period. This paper describes the background, design, sample size (approximately 6000 participants), and power estimates for the trial, as well as the treatment program and the rationale for the choice of inhaled bronchodilator. Plans for analysis of changes in pulmonary function parameters and for analysis of participants' survival and smoking-related morbidity are also discussed.

Administration, Inhalation↗
PubMed1988

In vitro mitogenesis of peripheral blood lymphocytes from rainbow trout (Salmo gairdneri).

1. In vitro mitogenesis of rainbow trout peripheral blood lymphocytes (RBT PBL) was investigated to assess the applicability of this procedure in assessment of fish health. The assay variables of media, mitogen type and concentration, serum supplementation, lymphocyte isolation procedure, and duration of incubation were assessed. 2. Concanavalin A (Con A) stimulated greater proliferation of RBT PBL than did lipopolysaccharide (LPS), phytohemagglutinin (PHA), or pokeweed mitogen (PWM). 3. RBT PBL, cultured with 10 micrograms Con A/ml and incubated for four or five days, exhibited greater proliferation than with other treatment combinations. 4. The degree of Con A-induced PBL proliferation varied significantly (P less than 0.05) among fish. The mean was positively correlated with the relative standard deviation and thus exhibited significant heteroscedasticity. 5. Human serum, as an alternative to FBS supplementation of the culture medium, did not enhance RBT PBL proliferation or reduce variation in mean proliferation. 6. Power analysis with variance estimates from this study reveal that sample size requirements of further studies under the given conditions could severely limit the applicability of this procedure for RBT health assessment. Further work in this area should center around standardization of culture conditions pertaining to the source of protein supplementation.

Animals↗
PubMed1991

Sources of T1 variance in normal human white matter.

The major factors contributing to T1 variance at 0.5 T in white matter were studied in healthy people. Anatomical location of the white matter sampled and differences between individuals contributed 74% of the total variance in serial measurements of the same subjects. There was also significant change over time within an individual subject that could not be attributed to machine drift. This information permitted estimates to be made concerning adequate sample size in future studies that examine for pathological white matter T1 change.

Adolescent↗
PubMed2006

Issues in the planning and conduct of non-randomised studies.

This paper discusses topics related to the planning and implementation of non-randomised clinical studies in orthopaedics. A well-conducted case-series is appropriate to demonstrate the safety of a surgical intervention. The case-series design involves the provision of a defined intervention to a group of patients with the ultimate objective of describing the final outcome, including such occurrences as complications. There is no alternative procedure serving as a control. The key aspects are to ensure enrolment of all eligible patients and to obtain a sufficiently large sample size to allow precise and valid estimation of complication risks. Targeted complications should be clearly defined and fully documented during a pre-defined follow-up period. Loss to follow-up should be minimised. Comparative studies are required to demonstrate treatment effectiveness. If a randomised controlled trial (RCT) is not feasible, an observational design such as a cohort or a case-control study should be considered. In observational designs, the treatment decision is made by the surgeons. In a case-control study, patients are selected based on their outcomes and their treatment or exposure status is recorded retrospectively. In a cohort study, groups of patients are selected based on their treatment and are followed for outcomes. There are numerous variations. Data can be collected prospectively or retrospectively; comparison groups may be concurrent or non-concurrent, or studied at different locations. The optimal design is tailored to clinical questions and research settings, while keeping in mind the respective methodological strengths and weaknesses of available options. The strength of the observational study is its proximity to daily clinical practice. The limitations are the possibility of numerous biases and confounding factors. Despite many challenges to the internal validity of non-randomised studies in orthopaedics surgery, it is possible to use such designs in order to provide reasonably valid answers to clinically important questions.

Algorithms↗
PubMed1999

Concepts of establishing clinical bioequivalence of chlorofluorocarbon and hydrofluoroalkane beta-agonists.

There are no established guidelines for judging equivalence between inhaled medications. The principles of establishing bioequivalence on the basis of bioavailability and pharmacokinetics may not be applicable to inhaled medications with predominantly topical and minimal systemic effects. For inhaled beta(2)-agonists, the most practical method of showing in vivo therapeutic equivalence is by comparing relative potencies (RPs) of pharmacodynamic effects (bronchodilation and bronchoprotection). A range of doses that includes placebo should be studied in an appropriate design with adequate sample size, and relative potency should be estimated. Hydrofluoroalkane and chlorofluorocarbon salbutamol are bioequivalent for both their bronchodilator (RP, 1.08; 90% confidence interval, 0.95%, 1.23%) and bronchoprotective effects (RP, 1.08; 90% confidence interval, 0.81%, 1.46%) with similar safety profile. Eighteen subjects are required in a cross-over design to demonstrate bronchoprotective bioequivalence with a confidence interval of 67% to 150% for the relative potency (80% power). For salbutamol, this can be achieved with a comparison of 100 and 200 microgram doses. Twelve subjects would suffice for a cumulative dose-response study for bronchodilator bioequivalence. For both outcomes, repeatability and quality control of measurements have to be ensured for an accurate interpretation of the results.

Adrenergic beta-Agonists↗
PubMed2002

Pedometer-determined ambulatory activity in individuals with type 2 diabetes.

This cross-sectional study presents the first normative data on pedometer-determined ambulatory activity, defined as steps/day, in 160 (98 males, 62 females; age=52.4 +/- 5.3 years; BMI=32.3 +/- 5.7) free-living individuals with type 2 diabetes. Participants took 6662 +/- 3077 steps per day, less than that reported in nondiabetic samples and more than that reported for samples living with more restrictive chronic conditions including claudication, joint replacement, chronic obstructive lung disease, and chronic heart failure. Steps/day and BMI were inversely and significantly correlated (r=-0.27, P<0.01). Further, there was a significant difference between BMI categories (from normal weight to obesity class III) with regard to steps/day (F=2.96, P<0.05). The difference was most apparent between the highest obesity classes (II and III) and normal weight categories. This data is useful for sample comparison purposes. In addition the standard deviation or variance estimates can be used to calculate samples sizes for intervention efforts. Objective quantification of ambulatory activity via simple and inexpensive pedometers permits researchers and practitioners to easily screen for level of activity along a continuum. This study opens the door for future research and clinical applications including identifying threshold values related to important health outcomes and evaluating incremental change due to various interventions in this population.

Body Mass Index↗
PubMed2003

Polymorphism in CYP1A1 and CYP2E1 genes and susceptibility to leukoplakia in Indian tobacco users.

Inter-individual genetic differences may contribute to differences in susceptibility to human diseases triggered by environmental exposures. In this study, we investigated polymorphisms at two sites in the CYP1A1 and three sites in the CYP2E1 genes in 99 leukoplakia patients and 227 controls from one Indian population. The frequencies of genotypes at these polymorphic sites (MspI and Ileu/Val) in the CYP1A1 and (PstI, RsaI and DraI) in the CYP2E1 genes, were similar in patient and control groups. But the combined rare and heterozygous genotypes (CC+CD) at the DraI site in the CYP2E1 gene were over-represented among patients compared with controls (age-adjusted odds ratio (OR)=2.02, 95% confidence interval (CI)=1.21-3.35). Light tobacco smokers (i.e. <21 pack-year) and light tobacco chewers (i.e. <104 chewing-year) with a "rare" C allele at the DraI site had high risk of leukoplakia (OR=2.88, 95% CI=1.16-7.22; OR=2.94, 95% CI=1.15-7.65, respectively). The "mixed tobacco" users with "rare" C allele are more susceptible to the disease than "exclusive" tobacco smokers and chewers. The results indicate that the "rare" C allele at the DraI polymorphic site in CYP2E1 gene may enhance susceptibility to leukoplakia among tobacco users in this population. But the low sample size limited the power to precisely estimate the tobacco-genotype interactions.

Adult↗
PubMed2000

Subject-by-formulation interaction in bioequivalence: conceptual and statistical issues. FDA Population/Individual Bioequivalence Working Group. Food and Drug Administration.

PURPOSE: The FDA has proposed replacing the current average bioequivalence criterion with population and individual bioequivalence criteria that consider variances in addition to the difference of averages. One of these variances in the individual bioequivalence criterion measures subject-by-formulation interaction, the extent to which the test-reference difference varies from person to person. This paper discusses conceptual and statistical issues raised in various publications and presentations with respect to the presence and estimation of such an interaction. METHODS: We focus on the importance of subject-by-formulation interaction, an understanding of what is a large interaction, and the assessment of the magnitude of this interaction in bioequivalence studies. Simulation studies, examples from the literature, and data from FDA files are used to demonstrate the magnitude of the interaction and its distribution under various conditions. RESULTS: The concept of a large interaction is tied to the concept of a large mean difference. We suggest that an interaction greater than 0.15 is a conservative criterion for a large interaction. Magnitudes of estimated interaction are affected by variability, sample size, and the selection of data sets that pass average bioequivalence. CONCLUSIONS: Examples of substantial interactions are beginning to appear. More data is needed before reaching definitive conclusions regarding the frequency and importance of observed interactions.

Chemistry, Pharmaceutical↗
PubMed1999

Expressing estimators of expected quality adjusted survival as functions of Nelson-Aalen estimators.

Quality adjusted survival has been increasingly advocated in clinical trials to be assessed as a synthesis of survival and quality of life. We investigate nonparametric estimation of its expectation for a general multistate process with incomplete follow-up data. Upon establishing a representation of expected quality adjusted survival through marginal distributions of a set of defined events, we propose two estimators for expected quality adjusted survival. Expressed as functions of Nelson-Aalen estimators, the two estimators are strongly consistent and asymptotically normal. We derive their asymptotic variances and propose sample-based variance estimates, along with evaluation of asymptotic relative efficiency. Monte Carlo studies show that these estimation procedures perform well for practical sample sizes. We illustrate the methods using data from a national, multicenter AIDS clinical trial.

Acquired Immunodeficiency Syndrome↗
PubMed1999

Long-term survival after ruthenium plaque radiotherapy for uveal melanoma. A meta-analysis of studies including 1,066 patients.

PURPOSE: Most case series of patients managed by brachytherapy for uveal melanoma are small and survival data show considerable inter-study variation. The aim of this study was to summarise similarly structured case series by meta-analysis. METHODS: A systematic review generated 5 similarly structured case series including survival data for 1,066 patients treated by ruthenium plaque radiotherapy for uveal melanoma. After assessing the inter-study clinical heterogeneity, data were weighed for study size and pooled. RESULTS: Patient and radiotherapy characteristics were largely homogenous, but tumour size varied considerably between studies. The 5-year melanoma-related mortality rate was 6% for small and medium tumours (T1/T2) and 26% for large (T3) tumours. The 5-year and 10-year melanoma-related mortality rates for a balanced set of tumours with small, medium and large tumours being present in similar proportions were 14% and 22%, respectively. CONCLUSION: Case series of patients with uveal melanoma managed by brachytherapy may be pooled to increase sample size and study power. The present estimate of survival following ruthenium plaque radiotherapy compares favourably with previously summarised data of survival after enucleation for similarly sized tumours.

Brachytherapy↗
PubMed2002

Quantitative risk assessment of durable glass fibers.

This article presents a quantitative risk assessment for the theoretical lifetime cancer risk from the manufacture and use of relatively durable synthetic glass fibers. More specifically, we estimate levels of exposure to respirable fibers or fiberlike structures of E-glass and C-glass that, assuming a working lifetime exposure, pose a theoretical lifetime cancer risk of not more than 1 per 100,000. For comparability with other risk assessments we define these levels as nonsignificant exposures. Nonsignificant exposure levels are estimated from (a) the Institute of Occupational Medicine (IOM) chronic rat inhalation bioassay of durable E-glass microfibers, and (b) the Research Consulting Company (RCC) chronic inhalation bioassay of durable refractory ceramic fibers (RCF). Best estimates of nonsignificant E-glass exposure exceed 0.05-0.13 fibers (or shards) per cubic centimeter (cm3) when calculated from the multistage nonthreshold model. Best estimates of nonsignificant C-glass exposure exceed 0.27-0.6 fibers/cm3. Estimates of nonsignificant exposure increase markedly for E- and C-glass when non-linear models are applied and rapidly exceed 1 fiber/cm3. Controlling durable fiber exposures to an 8-h time-weighted average of 0.05 fibers/cm3 will assure that the additional theoretical lifetime risk from working lifetime exposures to these durable fibers or shards is kept below the 1 per 100,000 level. Measured airborne exposures to respirable, durable glass fibers (or shards) in glass fiber manufacturing and fabrication operations were compared with the nonsignificant exposure estimates described. Sampling results for B-sized respirable E-glass fibers at facilities that manufacture or fabricate small-diameter continuous-filament products, from those that manufacture respirable E-glass shards from PERG (process to efficiently recycle glass), from milled fiber operations, and from respirable C-glass shards from Flakeglass operations indicate very low median exposures of 0, 0.0002, 0.007, 0.008, and 0.0025 fibers (or shards)/cm3, respectively using the NIOSH 7400 Method ("B" rules). Durable glass fiber exposures for various applications must be well characterized to ensure that they are kept below nonsignificant levels (e.g., 0.05 fibers/cm3) as defined in this risk assessment.

Air Pollutants, Occupational↗
PubMed2001

Design and analysis of trials with quality of life as an outcome: a practical guide.

Health Related Quality of Life (HRQoL) measures are becoming more frequently used in clinical trials, as both primary and secondary endpoints. Investigators are now asking statisticians for advice on how to plan (e.g., sample size) and analyze studies using HRQoL measures. HRQoL measures such as the SF-36 are usually measured on an ordered categorical (ordinal) scale. In the designing stages and when analyzing, the scales are often scored and the scores treated as if they were continuous and normally distributed. However the ordinal scaling of HRQoL measures leads to problems in determining sample size, and conventional parametric methods of estimation and hypothesis testing may not be appropriate for such outcomes. We present practical guidelines for the design and analysis of trials with HRQoL measures as outcomes. We used conventional statistical methods (i.e., t-tests and multiple regression), various ordinal regression models (proportional odds, continuation ratio, polytomous and stereotype) and bootstrap methods to analyze an HRQoL dataset. To illustrate the various methods we used HRQoL data on the SF-36 Role Limitations Emotional dimension for two groups of patients with leg ulcers. The bootstrap, t-test, and multiple regression methods gave similar results. The various ordinal regression models also gave similar results. If the HRQoL measure has a large number of ordered categories, most of which are occupied, and the underlying scale really is continuous but measured imperfectly by an instrument with a limited number of discrete values, then an informal rule of thumb is that this discrete scale should be treated as continuous if it has seven or more categories and as ordinal otherwise.

Algorithms↗
PubMed2006

Adaptive design: estimation and inference with censored data in a semiparametric model.

In this article, we provide a method of estimation for the treatment effect in the adaptive design for censored survival data with or without adjusting for risk factors other than the treatment indicator. Within the semiparametric Cox proportional hazards model, we propose a bias-adjusted parameter estimator for the treatment coefficient and its asymptotic confidence interval at the end of the trial. The method for obtaining an asymptotic confidence interval and point estimator is based on a general distribution property of the final test statistic from the weighted linear rank statistics at the interims with or without considering the nuisance covariates. The computation of the estimates is straightforward. Extensive simulation studies show that the asymptotic confidence intervals have reasonable nominal probability of coverage, and the proposed point estimators are nearly unbiased with practical sample sizes.

Antineoplastic Combined Chemotherapy Protocols↗