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PubMed2002

Maximum likelihood estimation in random effects cure rate models with nonignorable missing covariates.

We introduce a method of parameter estimation for a random effects cure rate model. We also propose a methodology that allows us to account for nonignorable missing covariates in this class of models. The proposed method corrects for possible bias introduced by complete case analysis when missing data are not missing completely at random and is motivated by data from a pair of melanoma studies conducted by the Eastern Cooperative Oncology Group in which clustering by cohort or time of study entry was suspected. In addition, these models allow estimation of cure rates, which is desirable when we do not wish to assume that all subjects remain at risk of death or relapse from disease after sufficient follow-up. We develop an EM algorithm for the model and provide an efficient Gibbs sampling scheme for carrying out the E-step of the algorithm.

Journal Article↗

PubMed1996

Estimating single-channel kinetic parameters from idealized patch-clamp data containing missed events.

We present here a maximal likelihood algorithm for estimating single-channel kinetic parameters from idealized patch-clamp data. The algorithm takes into account missed events caused by limited time resolution of the recording system. Assuming a fixed dead time, we derive an explicit expression for the corrected transition rate matrix by generalizing the theory of Roux and Sauve (1985, Biophys. J. 48:149-158) to the case of multiple conductance levels. We use a variable metric optimizer with analytical derivatives for rapidly maximizing the likelihood. The algorithm is applicable to data containing substates and multiple identical or nonidentical channels. It allows multiple data sets obtained under different experimental conditions, e.g., concentration, voltage, and force, to be fit simultaneously. It also permits a variety of constraints on rate constants and provides standard errors for all estimates of model parameters. The algorithm has been tested extensively on a variety of kinetic models with both simulated and experimental data. It is very efficient and robust; rate constants for a multistate model can often be extracted in a processing time of approximately 1 min, largely independent of the starting values.

Algorithms↗

PubMed2003

Genetic association tests for family data with missing parental genotypes: a comparison.

We consider three tests for genetic association in data from nuclear families (the Family-Based Association Test (FBAT) test proposed by Rabinowitz and Laird ([2000] Hum. Hered. 50:211-223), a second test proposed by Rabinowitz ([2002] J. Am. Stat. Assoc. 97:742-758), and the Family Genotype Analysis Program (FGAP) nonfounder or partial score test proposed by Clayton ([1999] Am. J. Hum. Genet. 65:1170-1177) and Whittemore and Tu ([2000] Am. J. Hum. Genet. 66:1329-1340)). We show that each test statistic arises from the efficient score of the family data as the solution to a set of constraints on its null expectation. Moreover, the FBAT and Rabinowitz tests (but not the FGAP test) are locally the most powerful among all tests satisfying their constraints. We used simulations to examine how the three tests perform in situations when their assumptions are violated and the number of families is not huge. We found that the FBAT test tended to have less power than the other two tests, particularly when applied to families in whom all offspring were affected. The Rabinowitz and FGAP tests performed similarly, although the latter tended to extract more information from families containing one typed parent. While none of the tests showed good power to detect rare, recessively acting genes, the Rabinowitz test with a sample variance estimate performed particularly poorly in this case. However, the Rabinowitz test with a model-based variance had power comparable to that of the FGAP test, and more accurate type I error rates. We conclude that for the situations we considered, the Rabinowitz test with model-based variance has good power without forfeiting robustness against misspecification of parental genotype probabilities. However, its utility is limited by the lack of a simple algorithm to apply it to families with varying structures and phenotypes.

Family↗

PubMed1988

Smoking cessation studies: a methodological comparison.

A wide variety in outcome criteria hinders comparison of results between smoking cessation studies. Three important methodological issues are discussed: analysis of data of participants who drop out of therapy, treatment of missing data, and repeated use of significance tests. These issues determine to a great extent the results of evaluation studies. In general, they are of interest to all researchers of addiction who study the effects of interventions. Several ways to decide on these issues and the consequences of these decisions are considered. Little consensus exists about the criterion for dropout. It is concluded that a dropout criterion is a burden rather than a help. A better criterion would be the number of sessions present. Few satisfying techniques exist to handle the problem of missing data. Evaluation studies need to set a priori standards to counter the increased risk of a Type I error, caused by the repeated use of significance tests. Reviewers need to be aware of the variety in data treatment before comparing results.

Behavior Therapy↗

PubMed1995

A simulation study of estimators for rates of change in longitudinal studies with attrition.

Many longitudinal studies and clinical trials are designed to compare rates of change over time in one or more outcome variables in several groups. Most such studies have incomplete data because some patients drop out before completing the study. The missing data may induce bias and inefficiency in naive estimates of important parameters. This paper uses Monte Carlo methods to compare the bias and efficiency of several two-stage estimators of the effect of treatment on the mean rate of change when the missing data arise from one of four processes. We also study the validity of confidence intervals and the power of hypothesis tests based on these estimates and their standard errors. In general, the weighted least squares estimator does relatively well, as does an analysis of covariance type estimator proposed by Wu et al. The best estimates of variance components are based on complete cases or maximum likelihood.

Analysis of Variance↗

PubMed2001

Plasma exchange for Guillain-Barré syndrome.

BACKGROUND: Guillain-Barré syndrome is an acute symmetric usually ascending and usually paralysing illness due to inflammation of peripheral nerves. It is thought to be caused by autoimmune factors, such as antibodies. Plasma exchange removes antibodies and other potentially injurious factors from the blood stream. It involves connecting the patient's blood circulation to a machine which exchanges the plasma for a substitute solution, usually albumin. Several studies have evaluated plasma exchange for Guillain-Barré syndrome. OBJECTIVES: To systematically review the evidence concerning the efficacy of plasma exchange for treating Guillain-Barré syndrome. SEARCH STRATEGY: Search of the Cochrane Neuromuscular Disease Trial Register for randomised trials concerning plasma exchange in Guillain-Barré syndrome, search of the bibliographies of identified papers and enquiry from the authors of the papers. SELECTION CRITERIA: Randomised and quasi-randomised trials of plasma exchange versus sham exchange or supportive treatment. DATA COLLECTION AND ANALYSIS: Potentially relevant papers were scrutinised by two reviewers and the selection of eligible studies was agreed by them and a third reviewer. Data were extracted by one reviewer and checked by a second reviewer. Some missing data were obtained from the authors of studies. MAIN RESULTS: Six eligible trials concerning 649 patients were identified, all comparing plasma exchange versus supportive treatment alone. Primary outcome measures ~Bullet~Time to recover walking with aid In the only two trials for which this measure was reported the median time to recover this ability was faster in the plasma exchange than the control group. ~Bullet~Time to onset of motor recovery in mildly affected patients In the one trial for which this measure was available the time was significantly shortened in the plasma exchange group. Secondary outcome measures ~Bullet~Improvement in disability grade at 4 weeks In five trials, there were significantly more patients who had improved by one disability grade or more in the plasma exchange group as compared to the control group. Patients treated with plasma exchange fared significantly better in the following secondary outcome measures: time to recover walking without aid, percentage of patients requiring artificial ventilation, duration of ventilation, full muscle strength recovery after one year, and severe sequelae after one year. There were less patients with infectious events and cardiac arrhythmias in the plasma exchange than the control group. Subgroup analyses Plasma exchange was beneficial in patients with mild, moderate and severe (needing ventilation) Guillain-Barré syndrome. It was beneficial in patients with a disease duration of seven or less days and also in those with disease lasting more than seven days. However, in the only trial that enrolled patients up to 30 days from disease onset, the benefit of plasma exchange in patients treated after seven days was less apparent. Type of treatment Single studies showed that two plasma exchanges were significantly superior to none for mild Guillain-Barré syndrome and four to two for moderate Guillain-Barré syndrome but that six were not superior to four for severe Guillain-Barré syndrome requiring ventilation. One study suggested that continuous flow plasma exchange was significantly superior to intermittent flow. Another study found no significant difference between the two techniques. The same study found a significantly higher rate of adverse events with fresh frozen plasma as the replacement fluid than albumin. REVIEWER'S CONCLUSIONS: Plasma exchange is the first and only treatment that has been proven to be superior to supportive treatment alone in Guillain-Barré syndrome. Consequently, plasma exchange should be regarded as the treatment against which new treatments, such as intravenous immunoglobulin, should be judged. In mild Guillain-Barré syndrome two sessions of plasma exchange are superior to none. In moderate Guillain-Barré syndrome four sessions are superior to two. In severe Guillain-Barré syndrome six sessions are no better than four. Continuous flow plasma exchange machines may be superior to intermittent flow machines and albumin to fresh frozen plasma as the exchange fluid. Plasma exchange is more beneficial when started within seven days after disease onset rather than later, but was still beneficial in patients treated up to 30 days after disease onset. The value of plasma exchange in children less than 12 years old is not known.

Guillain-Barre Syndrome↗

PubMed2002

Complete imputation of missing repeated categorical data: one-sample applications.

Longitudinal studies with repeated measures are often subject to non-response. Methods currently employed to alleviate the difficulties caused by missing data are typically unsatisfactory, especially when the cause of the missingness is related to the outcomes. We present an approach for incomplete categorical data in the repeated measures setting that allows missing data to depend on other observed outcomes for a study subject. The proposed methodology also allows a broader examination of study findings through interpretation of results in the framework of the set of all possible test statistics that might have been observed had no data been missing. The proposed approach consists of the following general steps. First, we generate all possible sets of missing values and form a set of possible complete data sets. We then weight each data set according to clearly defined assumptions and apply an appropriate statistical test procedure to each data set, combining the results to give an overall indication of significance. We make use of the EM algorithm and a Bayesian prior in this approach. While not restricted to the one-sample case, the proposed methodology is illustrated for one-sample data and compared to the common complete-case and available-case analysis methods.

Algorithms↗

PubMed1996

Performance of multi-layer feedforward neural networks to predict liver transplantation outcome.

A novel multisolutional clustering and quantization (MCQ) algorithm has been developed that provides a flexible way to preprocess data. It was tested whether it would impact the neural network's performance favorably and whether the employment of the proposed algorithm would enable neural networks to handle missing data. This was assessed by comparing the performance of neural networks using a well-documented data set to predict outcome following liver transplantation. This new approach to data preprocessing leads to a statistically significant improvement in network performance when compared to simple linear scaling. The obtained results also showed that coding missing data as zeroes in combination with the MCQ algorithm, leads to a significant improvement in neural network performance on a data set containing missing values in 59.4% of cases when compared to replacement of missing values with either series means or medians.

Algorithms↗

PubMed1995

A multiple imputation strategy for clinical trials with truncation of patient data.

Clinical trials of drug treatments for psychiatric disorders commonly employ the parallel groups, placebo-controlled, repeated measure randomized comparison. When patients stop adhering to their originally assigned treatment, investigators often abandon data collection. Thus, non-adherence produces a monotone pattern of unit-level missing data, disabling the analysis by intent-to-treat. We propose an approach based on multiple imputation of the missing responses, using the approximate Bayesian bootstrap to draw ignorable repeated imputations from the posterior predictive distribution of the missing data, stratifying by a balancing score for the observed responses prior to withdrawal. We apply the method and some variations to data from a large randomized trial of treatments for panic disorder, and compare the results to those obtained by the original analysis that used the standard (endpoint) method.

Adult↗

PubMedDate not supplied

Ensuring the quality of quality of life data: the Southwest Oncology Group experience.

The Southwest Oncology Group (SWOG) has successfully included quality of life (QOL) questionnaires in selected oncology treatment trials. Extensive quality control procedures have been necessary for obtaining and maintaining good questionnaire submission rates. Since the first QOL study was activated in SWOG in 1990, the Group has found it increasingly necessary to incorporate centralized monitoring of the QOL assessment schedule. Successful quality control strategies are presented. Current submission rates for five SWOG phase III treatment trials (both open and closed) and one chemoprevention trial are presented for those scheduled QOL assessments for which we have follow-up data. Reasons for missing QOL questionnaires and the extent of missing data within submitted QOL questionnaires are described for two different disease contexts: a trial for patients with advanced stage disease, and a trial for patients with earlier stage disease.

Clinical Trials as Topic↗

PubMed2001

Pharmacological treatment for aphasia following stroke.

BACKGROUND: Aphasia describes language impairment associated with a brain lesion. OBJECTIVES: The objective of this review was to assess the effects of drugs on language abilities when given to people with aphasia following stroke. SEARCH STRATEGY: We searched the Cochrane Stroke Group Register (last searched: May 2001), and reference lists of relevant articles to December 1998. We also contacted academic institutions and other researchers to identify further published and unpublished trials. MEDLINE was searched from 1966-1998, and CINAHL from 1982-1998. We searched the International Journal of Disorders of Communication by hand (known by other names in the past), from 1969 to 1998. SELECTION CRITERIA: Randomised controlled trials comparing: ~bullet~Any drug given to improve language, versus no treatment, or versus placebo ~bullet~Any drug given to improve language versus speech and language therapy ~bullet~One drug given to improve language versus another drug given with the same aim DATA COLLECTION AND ANALYSIS: The principal reviewer collected the data, and assessed the quality of the trials with independent data checking and methodological advice. If we could not perform a statistical combination of different studies, we sought missing data. Failing that we provided a description. We sought missing data from authors, or where appropriate, a drug company. MAIN RESULTS: We considered fifty two studies in detail, from which we identified ten trials suitable for the review. In most cases the methodological quality was unassessable, and only one trial reported sufficient detail for us to complete a description and analysis. This study did lose a large number of patients during its course. Drugs used in the trials identified were piracetam, bifemalane, piribedil, bromocriptine, idebenone, and Dextran 40. We found weak evidence that patients were more likely to have improved on any language measure at the end of the trial if they had received treatment with piracetam (odds ratio 0.46, 95% confidence interval 0.3 to 0.7). The evidence is considered weak because of the large numbers of drop outs from the trials identified, who were lost to follow up. Patients who were treated with piracetam were no more likely (considering statistical significance) than those who took a placebo to experience unwanted effects, including death (odds ratio 1.29, 95% confidence interval for difference 0.9 to 1.7). However, the differences in death rates between the two groups, even though not not statistically significant, do give rise to some concerns that there may be an increased risk of death from taking piracetam. We could not determine if drug treatment is more effective than speech and language therapy. We could not determine whether one drug is more effective than another. REVIEWER'S CONCLUSIONS: The main conclusion of this review is that drug treatment with piracetam may be effective in the treatment of aphasia after stroke. Further research is needed to explore the effects of drugs for aphasia, in particular piracetam. If a trial is done, this must be large enough to have adequate statistical power. The safety of the drug should be of primary interest. Researchers should examine the long term effects of this treatment, and whether it is more effective than speech and language therapy.

Aphasia↗

PubMed2000

Home care by outreach nursing for chronic obstructive pulmonary disease.

BACKGROUND: Chronic obstructive pulmonary disease (COPD) is characterised by progressive airflow obstruction, worsening exercise performance and deterioration in health. It is associated with significant morbidity, mortality and costs to health care systems. Care strategies, such as outreach nursing in the community, may reduce this burden. OBJECTIVES: To evaluate the effectiveness of outreach respiratory health care worker programmes for patients with COPD in terms of improving lung function, exercise tolerance and health related quality of life (HRQL) of patient and carer, and reducing mortality and hospital service utilisation. SEARCH STRATEGY: A search was carried out using the Cochrane Airways Group database. Bibliographies of identified RCTs were searched for additional relevant RCTs. Authors of identified RCTS were contacted for other published and unpublished studies. SELECTION CRITERIA: Only randomised control trials of patients with COPD were included. The intervention was an outreach nurse visiting patients in their homes, providing support, education, monitoring patient status and providing liaison with physicians. Interventions that used nurse practitioners who provided therapeutic intervention were also included. Studies in which the therapeutic intervention under test was physical training were not included. DATA COLLECTION AND ANALYSIS: Data extraction and study quality assessment were performed independently by two reviewers. Where further or missing data was required, authors of studies were contacted. MAIN RESULTS: Four studies were found. Three assessed mortality following twelve months of care (n=96, 152 and 301), and one after seven months(n=75). Meta-analysis demonstrated that mortality was not significantly reduced by the intervention, Peto Odds Ratio 0.72; 95 % confidence interval 0.43, 1.21. Post hoc subgroup analysis suggested that mortality was reduced by the outreach nursing programme in patients with less severe disease. Significant improvements in health related quality of life were reported in one study in moderate COPD, but not in a study in patients with severe disease. No changes in lung function or exercise performance were found in the studies where data were available. Hospital admissions were reported in only one study in patients with severe disease and no benefit was observed. REVIEWER'S CONCLUSIONS: Patients with moderate COPD may have mortality and health related quality of life gains from a nursing outreach programme, but there are no data about reductions in hospital utilisation. Patients with severe COPD do not appear to have benefit from such programmes and one large study found no reduction in hospital admissions in such patients.

Community-Institutional Relations↗

PubMed2000

Long-acting beta2-agonists for chronic obstructive pulmonary disease.

BACKGROUND: Chronic obstructive pulmonary disease (COPD) is characterised by airflow limitation which is only partially reversible. Long acting beta2-agonists, effective in the management of asthma,are also recommended for COPD management so it is important to establish whether these drugs are effective in reducing COPD symptoms in view of the potential side effect and cost burden. OBJECTIVES: To determine the effectiveness of long acting beta2-adrenoceptor agonists in improving lung function and quality of life and reducing dyspnoea in patients with COPD. SEARCH STRATEGY: A search was carried out using the Cochrane Airways Group register. Bibliographies of identified RCTs were searched for additional relevant RCTs. Authors of identified RCTs were contacted for other published and unpublished studies. In addition, unpublished studies were also obtained from the pharmaceutical companies that manufacture long acting beta2- adrenoceptor agonists. SELECTION CRITERIA: All randomised controlled trials over four weeks in duration comparing treatment with long-acting beta2-adrenoceptor agonists (salmeterol and formoterol) with placebo in patients with stable non-reversible COPD. Outcome measures included forced expiratory volume in one second (FEV1), peak expiratory flow rate (PEFR), symptom scores, six minute walk distance, quality of life scores, Borg scores for dyspnoea and rescue bronchodilator use. DATA COLLECTION AND ANALYSIS: Data extraction and study quality assessment was performed independently by two reviewers. Where further or missing data were required, authors of studies were contacted. MAIN RESULTS: Thirty three abstracts were identified as potentially relevant. Of these, four randomised controlled trials (RCTs) were included in this review. Two were parallel group studies of 16 week duration and two were cross-over studies with four week treatment arms. All four RCTs assessed the efficacy of salmeterol in COPD. In a 16 week study of salmeterol 50 mcg and 100 mcg twice daily treatment, a significant increase in FEV1 was seen in both treatment groups. The weighted mean difference (WMD) for the increase in FEV1 for the 50 mcg group was 0.10 litre (95% CI: 0.05;0. 15) and in the 100 mcg group the WMD was 0.12 litre (95% CI: 0.06; 0. 17 ). In the two cross-over studies of four weeks treatment, salmeterol 50 mcg twice daily treatment did not show significant increases in FEV1 (WMD = 0.04 litre, 95% CI: -0.06; 0.15). Similarly, morning and night time PEFR was not significantly improved with salmeterol treatment. In a 16 week study, disease-specific quality of life, measured using the St. George's Respiratory Questionnaire (SGRQ), showed a significant improvement after 50 mcg twice daily, but not after 100 mcg twice daily. This improvement exceeded the threshold for a clinically significant change with this questionnaire. General health status, as measured by the Medical Outcomes Short Form 36, did not improve in any of the eight components with either salmeterol dose. No significant difference was demonstrated in the mean change from baseline in the six minute walk distance (WMD = 1.9 metres, 95% CI: -15.4;19.3). Breathlessness was reduced in one study in patients receiving salmeterol 50 mcg twice daily group. Significantly more patients in this group had Borg scores for breathlessness less than three (a score of three indicating moderate dyspnoea) compared to the placebo treated group (Peto Odds Ratio = 0.60, 95% CI: 0.40; 0.88). Neither dose of salmeterol influenced the incidence of COPD exacerbations, (50 mcg: Peto Odds Ratio = 0.74, 95% CI: 0.47, 1.15) and (100 mcg: Peto Odds Ratio = 0.98, 95% CI: 0.64, 1.52). REVIEWER'S CONCLUSIONS: Treatment of patients with COPD with long acting beta2-agonists produces only small increases in FEV1. In one study, a dose of salmeterol 50 mcg twice daily produced a reduction in breathlessness and a clinically significant improvement in quality of life. (ABSTRACT TRUNCATED)

Adrenergic beta-Agonists↗

PubMed2000

Short-acting beta 2 agonists for stable chronic obstructive pulmonary disease.

BACKGROUND: Chronic Obstructive Pulmonary Disease (COPD) is a chronic condition characterised by progressive airflow limitation that is at most partially reversible. Despite the lack of reversibility patients often report symptomatic improvement with short-acting beta 2 bronchodilator medications. They are used on either an "as required" or a "regular plus as required basis" and they may be used in conjunction with other bronchodilator medicines such as ipratropium and methylxanthines. These medicines are used in the management of both stable and acute exacerbations of COPD. This review examined the effect of short-acting beta 2 bronchodilators given by inhalation in stable COPD. OBJECTIVES: To determine the clinical effect and assess the adverse effects of inhaled short-acting beta 2 agonist bronchodilators compared with placebo in stable COPD. SEARCH STRATEGY: Only randomised controlled trials (RCTs) were considered. RCTs were identified using the Cochrane Airways Group database (CENTRAL). In addition, the reference lists of review articles and RCTs retrieved in full were searched for other potentially relevant citations. SELECTION CRITERIA: Only trials with adult patients with stable COPD, as defined by internationally accepted guidelines (ATS, ERS or BTS) were included in this review. All trials had a minimum duration of 7 days of regular treatment with short-acting beta 2 bronchodilators given by inhalation and compared with placebo. Data from trials where beta 2 agonists were used alone or in combination with other medicines (e.g. ipratropium bromide) were used only if there was a direct comparison between beta 2 bronchodilator alone and placebo. DATA COLLECTION AND ANALYSIS: Outcomes were analysed as continuous or dichotomous outcomes, using standard statistical techniques. For continuous outcomes, the weighted mean difference (WMD) and 95% confidence intervals were calculated and for dichotomous outcomes, the odds ratio was calculated with 95% confidence intervals by Peto's methods. Funnel plots were used to test for publication bias. MAIN RESULTS: Thirteen studies were included in this review. Most had small sample sizes and some of the sutides used very short-acting outdated compounds. All the studies used a cross-over design and were of high quality. Spirometry done at the end of study period was measured after administration of treatment (post-bronchodilator) which showed both FEV1 (0.150 L/min, 95%CI: 0. 02-0.28) and FVC (0.310 L, 95%CI: 0.00-0.62) to improve significanly but slightly when compared to placebo. A few studies measured FRC, airway resistance or conductance at the end of the study period. In all cases these measurements were done several hours after treatment, and no significant differences (p>0.05 in all cases) were found between the bronchodilator and placebo groups. Walking test Large increases in 6MW distance was observed in two studies, however one study did not show any positive improvements. There was a large increase in the 12MW distance as shown by one study. Due to the small number of studies reporting this outcome no significant differences were found in the walking distance between the bronchodilator and placebo groups. Peak Flow Rate Both morning (36. 04 L/min; 95%CI: 0.80-71.27) and evening (36.68 L/min; 95%CI: 2. 47-70.89) PEFR were significantly higher during active treatment than during placebo. Symptoms Breatlessness was measured on various scales therefore data that were presented in a suitable form were combined using standardized means for inclusion in the analysis. A significant improvement (-0.33; 95%CI: -0.58 to -0.07 with p=0.01) in the breathlessness score was observed during treatment with beta-2 agonist when compared to placebo. Cough was reported to improve significantly (data not usable) during treatment with beta2 agonist in one study but not in two others. A non-significant decrease in sputum production was reported by Wilson 1980, however four other studies reported no

Administration, Inhalation↗

PubMed1993

The geo-systemic model and mortality in Ubon-Ratchathani.

The Thailand 'Rural Data Base' includes a number of variables in the social, economic, services, housing, agriculture and health fields. This data base comes from a survey at the village level. There are weaknesses in the data base--inaccuracies, missing data, etc ... that make it difficult to discover the structures that theoretically exist in the data using direct analysis methods, like correlation analysis for instance. The 2383 villages of the province of Ubon-Ratchathani, in Thailand, are the units of this study. The objective is to uncover the hypothetical health disparities that exist among the villages and to devise a typology of villages that will show classes of villages that have significant problems and, also, classes of villages that could suggest concrete solutions and be used as models. Some variables of the data base are considered dependent (health indicators) and others are considered independent; they are used to build a model called 'geo-systemic' that is used to expose mortality differentials in Ubon-Ratchathani.

Adolescent↗

PubMed1997

Does organization matter? A multilevel analysis of the demand-control model applied to human services.

The demand-control model (DC model) in occupational epidemiology suggests that health, an individual attribute, is partly determined by work organization, via the interplay of demand and control, job strain. The objective of this study was empirical assessment of the model's tenet of an organizational determination of individual health. An emerging analytic method, multi-level modelling, permits such an assessment. The study encompasses two large Swedish human service organizations. It was based on a nationally representative sample of 291 local organizational units (level 2) with 8296 employees (level 1), a median of 18 employees per unit. 5730 persons (69.1%) completed the questionnaire. Listwise deletion of missing data left a net study base of 4756 individuals in 284 units. Missing data were largely random. Demand and control were measured by standard questions and combined into a job strain index. Two such indices were calculated, one for quantitative demands and one for emotional demands. Individual attributes included age, gender, marital status, having children, social anchorage, and education. There were two dependent variables, self-assessed psychovegetative symptoms (worry, anxiousness, sadness, sleep difficulties, restlessness, and tension) and exhaustion (fatigue, feelings of being used up and overworked), both measured as summative indices. For psychovegetative health, a null model yielded 2.2% level 2 variance, unchanging when individual attributes were included in a random intercepts model. Inclusion of the strain variables rendered level 2 variance non-significant, decreasing level 1 variance by 23% and level 2 variance by 62%. For exhaustion, level 2 variation was 8.3% in the null model and 1.6% in the final model, with strain variables. The strain variables utilized in the DC-model thus draw a substantial part of their variation from the organizational level. It is concluded that the claim of the DC model to rely on organizational factors receives support.

Adult↗

PubMedDate not supplied

Statistical approaches to experimental design and data analysis of in vivo studies.

The objective of any experiment is to obtain an unbiased and precise estimate of a treatment effect in an efficient manner. Statistical aspects of the design, conduct, and analysis of the experiment play a major role in determining whether this goal is met. We highlight some of the more important statistical issues that pertain to in vivo studies. Particular emphasis is placed on the role of randomization, the number of animals, the utilization of repeated measures data, adjustments for missing data, and dealing with multiple causes of death or treatment failure. The discussion is not intended to be a comprehensive guide to all the statistical issues that can occur in animal experiments. Rather, the objective is to acquaint researchers with components of the experiment that will require careful statistical thought.

Animals↗

PubMed1998

Multiple imputation for early stopping of a complex clinical trial.

It is desirable to have procedures available for stopping a clinical trial early if there appears to be no treatment effect. Conditional power procedures allow for early stopping in favor of the null hypothesis if the probability of rejecting H0 at the planned end of the trial given the current data and a value of the parameter of interest is below some threshold level. Lan, Simon, and Halperin (1982, Communications in Statistics C1, 207-219) proposed a stochastic curtailment procedure that calculates the conditional power under the alternative hypothesis. Alternatively, predictive power procedures incorporate information from the observed data by averaging the conditional power over the posterior distribution of the parameter. For complex problems in which explicit evaluation of conditional power is not possible, we propose treating the problem of projecting the outcome of a trial given the current data as a missing data problem. We then complete the data using multiple imputation and thus eliminate the need for explicit calculation of conditional power. We apply this method to AIDS Clinical Trials Group (ACTG) protocol 118 and to several simulated clinical trials.

Anti-HIV Agents↗