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PubMed1990

Interdisciplinary approach to assessing the health risk of air toxic chemicals: an overview.

To assist the regulatory branch of the Environmental Protection Agency in addressing the risk assessment of air toxics, the Health Effects Research Laboratory initiated a comprehensive inhalation toxicology program to provide key health effects data missing from the current data base. A priority ranking of chemicals based on the potential for substantial human exposure and the need for health effects data was developed to identify candidate chemicals for toxicological research. The major goal of the program is to evaluate the concentration-response from acute, intermittent and subchronic inhalation exposures to developmental, genetic, hepatic, immunologic, neurologic, pulmonary and reproductive toxicity in a manner that provides data for the regulatory health assessment of air toxic chemicals. Extrapolation and dosimetry research is also conducted to improve the basis for human risk assessment. Determination of biological endpoints to be examined will be decided on a compound-by-compound basis, depending on the physical, chemical and structural characteristics of the chemical and evaluation of the existing health data base. Although the main emphasis is on inhalation as the primary route of exposure, some of the laboratories will compare inhalation to other routes, such as oral, to better understand the influence of route of exposure and hence the potential applicability of existing health data. Acute and intermittent exposures will be done for all compounds. Upon evaluation of the acute results, a decision will be made as to whether subchronic studies are needed. Endpoints that show unusual sensitivity may be investigated in greater detail. The total length of exposure will vary from 1 to 21 days. The daily length of exposure will range from 1 to 8 hr. If adverse effects are observed at ambient levels, the time to recovery after exposure will be investigated.

Administration, Inhalation↗
PubMed1994

[Molecular phylogenies and nucleotide insertion-deletion].

Molecular trees based on the analysis of sequence data can be obtained through parsimony procedures. Such approaches identify evolutionary events (homologies and homoplasies and their location in the tree). Nevertheless, it is not possible generally to analyze directly the information given by the aligned sequences. Noticeably, gaps ("indel") need special coding. Standard procedures for coding gaps offer alternatives which suffer of several weaknesses. In this article a new strategy of coding the sequences is defined in view to express the potential phylogenetic information contained in complex zones with internested insertion/deletion and substitutions, contrary to what is done up to the present. This strategy applies without loss or distortion of information to any case where gaps are present in aligned sequences. According to the hierarchy of internested states of characters (sites), this strategy introduces in the data matrix question marks, "?", which are optimized in fine in the cladogram based on all data. These "?" are not missing data, they are methodological codes, neutral to a priori phylogenetic hypotheses.

Base Sequence↗
PubMed1987

Hidden bias in the use of archival data.

Nonresponses in archival data may violate the missing-at-random assumption in ways difficult to detect. Standard methods of comparing sociodemographics of respondents and nonrespondents are inappropriate when the units of analysis are not also the individuals who maintain the archival record. Under these circumstances, the distribution of missing data may be correlated with the dependent variable and traits of the record keepers. This will distort relationships, especially when listwise deletion of missing values is used in multivariate analysis. Data are used from a large clinical chart study of mentally ill patients to demonstrate the process of identifying hidden bias and the implications of such bias.

Archives↗
PubMed2006

Analysis of the benefits of a Mediterranean diet in the GISSI-Prevenzione study: a case study in imputation of missing values from repeated measurements.

The problem of missing values has increasingly being recognized in epidemiology. New methods allow for the analysis of missing data that can provide valid estimates of epidemiological quantities of interest. The GISSI-Prevenzione study was aimed to reliably assess the long-term relationship between the consumption of foods typical of the Mediterranean diet and the risk of mortality amongst 11,323 Italians with prior myocardial infarction. Food intake frequencies were recorded repeatedly over the 4.5 years of follow-up and missing values affected each food variable at increasing rates over the course of the study. Comparisons were made between the results obtained from the analysis of the complete data and those obtained after imputing the missing data with simple imputation methods and with various implementations of the multiple imputation (MI) method. MI appeared to best address the issue of missing data on the food intake frequencies, preserving the observed distributions and relationships between variables whilst producing plausible estimates of variability. Given its theoretical properties and flexibility to different types of data, MI is more likely to provide valid estimates, compared to complete data analysis and imputation by simple methods, and is thus worthy of wider consideration amongst epidemiological researchers.

Confidence Intervals↗
PubMedDate not supplied

Why are missing quality of life data a problem in clinical trials of cancer therapy?

Assessment of health related quality of life has become an important endpoint in many cancer clinical trials. Because the participants of these trials often experience disease and treatment related morbidity and mortality, non-random missing assessments are inevitable. Examples are presented from several such trials that illustrate the impact of missing data on the analysis of QOL in these trials. The sensitivity of different analyses depends on the proportion of assessments that are missing and the strength of the association of the underlying reasons for missing data with disease and treatment related morbidity and mortality. In the setting of clinical trials of cancer therapy, the assumption that the data are missing completely at random (MCAR) and analyses of complete cases is usually unjustified. Further, the assumption of missing at random (MAR) may also be violated in many trials and models appropriate for non-ignorable missing data should be explored. Recommendations are presented to minimize missing data, to obtain useful documentation concerning the reasons for missing data and to perform sensitivity analyses.

Breast Neoplasms↗
PubMed1995

Methodological and statistical problems in sleep apnea research: the literature on uvulopalatopharyngoplasty.

A comprehensive review of the literature on the surgical treatment of sleep apnea found 37 appropriate papers (total n = 992) on uvulopalatopharyngoplasty (UPPP). Methodological and statistical problems in these papers included the following: 1) There were no randomized studies and few (n = 4) with control groups. 2) Median sample size was only 21.5; thus statistical power was low and clinically important associations were routinely classified as "not statistically significant". 3) Only one paper presented the confidence bounds that might distinguish between statistical and clinical significance. 4) Because of short follow-up time and infrequent repeat follow-ups, little is known about whether UPPP results deteriorate with time. 5) In at least 15 papers, bias caused by retrospective designs and nonrandom loss to follow-up raised questions about the generalizability of results. 6) Few papers associated polysomnographic data with patient-based quality of life measures. 7) Missing data and missing and inconsistent definitions were common. 8) Baseline measures were often biased because the same assessment was inappropriately but routinely used for both screening and baseline. We conclude that because of these and other problems, there is much that is needlessly unknown about UPPP. It is the responsibility of the research and professional communities to define training, editorial and review procedures that will raise the methodological and statistical quality of published research.

Humans↗
PubMed2000

Individual patient data meta-analysis of randomized anti-epileptic drug monotherapy trials.

Meta-analysis may be based on either aggregate data or individual patient data (IPD). Three reasons why IPD are desirable for the meta-analysis of anti-epileptic drug (AED) monotherapy trials are: (1) to undertake a more complete analysis of time-to-event outcomes; (2) to investigate the interaction between AED and type of epilepsy; and (3) to undertake re-analysis of the trial to obtain results for all relevant outcomes. We demonstrate that IPD meta-analysis is possible in AED research. Problems arose from missing data at four levels: (1) unknown trials; (2) known trials but no IPD supplied; (3) known trials but missing outcome data for some individuals within trials; and (4) known trials but missing covariate data for some individuals within trials. Empirical evidence of the reliability of meta-analyses based on aggregate rather than individual patient data is still lacking. Examples of other benefits such projects may bring include improvements to the design of a new trial in the area, in terms of the sample size considerations, the definition of outcomes and data collection.

Anticonvulsants↗
PubMed2005

A multistate Markov chain model for longitudinal, categorical quality-of-life data subject to non-ignorable missingness.

Quality-of-life (QOL) is an important outcome in clinical research, particularly in cancer clinical trials. Typically, data are collected longitudinally from patients during treatment and subsequent follow-up. Missing data are a common problem, and missingness may arise in a non-ignorable fashion. In particular, the probability that a patient misses an assessment may depend on the patient's QOL at the time of the scheduled assessment. We propose a Markov chain model for the analysis of categorical outcomes derived from QOL measures. Our model assumes that transitions between QOL states depend on covariates through generalized logit models or proportional odds models. To account for non-ignorable missingness, we incorporate logistic regression models for the conditional probabilities of observing measurements, given their actual values. The model can accommodate time-dependent covariates. Estimation is by maximum likelihood, summing over all possible values of the missing measurements. We describe options for selecting parsimonious models, and we study the finite-sample properties of the estimators by simulation. We apply the techniques to data from a breast cancer clinical trial in which QOL assessments were made longitudinally, and in which missing data frequently arose.

Antineoplastic Agents↗
PubMed2006

The use of missing birth record data as a marker for adverse reproductive outcomes: a geocoded analysis of birth record data.

Adverse reproductive outcomes (AROs) disproportionately affect black American infants and significantly contribute to the U.S. infant mortality rate. Without accurate understanding of AROs, there remains little hope of ameliorating infant mortality rates or eliminating infant health disparities. However, despite the importance of monitoring infant mortality rates and health disparities, birth record data quality is not assured. Racial disparities in the reporting of birth record data have been documented, and missing birth record data for AROs appears to be disproportionate. Due to the extent of missing birth record data, innovative strategies have been developed to evaluate relationships between maternal socioeconomic status (SES) and community-based ARO rates. Because addresses convey aggregate information about income level, education and occupation, ZIP codes, census tracts and census block-groups have been applied to geocoding efforts. The goals of this study are to: 1) analyze the extent of missing birth record data for New Jersey areas with high rates of an ARO (preterm birth), 2) evaluate associations between the extent of missing birth record data and other AROs, and 3) consider how geocoding strategies could be applied to provide a basis for understanding maternal SES risk factors and ARO resource allocation for at-risk communities.

Adult↗
PubMed1998

Nonlinear Time&hyphenSeries Prediction with Missing and Noisy Data

We derive solutions for the problem of missing and noisy data in nonlinear time&hyphenseries prediction from a probabilistic point of view. We discuss different approximations to the solutions &hyphen in particular, approximations that require either stochastic simulation or the substitution of a single estimate for the missing data. We show experimentally that commonly used heuristics can lead to suboptimal solutions. We show how error bars for the predictions can be derived and how our results can be applied to K&hyphenstep prediction. We verify our solutions using two chaotic time series and the sunspot data set. In particular, we show that for K&hyphenstep prediction, stochastic simulation is superior to simply iterating the predictor.

Journal Article↗
PubMed1999

Multiple imputation of missing blood pressure covariates in survival analysis.

This paper studies a non-response problem in survival analysis where the occurrence of missing data in the risk factor is related to mortality. In a study to determine the influence of blood pressure on survival in the very old (85+ years), blood pressure measurements are missing in about 12.5 per cent of the sample. The available data suggest that the process that created the missing data depends jointly on survival and the unknown blood pressure, thereby distorting the relation of interest. Multiple imputation is used to impute missing blood pressure and then analyse the data under a variety of non-response models. One special modelling problem is treated in detail; the construction of a predictive model for drawing imputations if the number of variables is large. Risk estimates for these data appear robust to even large departures from the simplest non-response model, and are similar to those derived under deletion of the incomplete records.

Aged↗
PubMed2006

Suicide in the Greek penal system and the problem of various limitations in relevant studies.

Suicides in prison are not merely self-destructive acts or a "cry for help." They reflect the inherent need for freedom and the repercussions of imprisonment. SPACE statistics on suicides in prison reveal a rate above 10 per 10,000 in 10 European countries, 4 of which have a rate above 20. Greek data do not appear in all SPACE statistics. This fact has stimulated the present paper. Unpublished data obtained from the Greek Ministry of Justice reveal that Greece belongs to the group of countries with a rate below 10 in 1995 (the year of SPACE statistics). However, the suicide rates fluctuated widely in Greece from a low rate of 3.2 per 10,000 prisoners (convicted, on remand, or hospitalized) in 1982 to the incredibly high rate of nearly 40 in the year 1979 (11 suicides, 10 of which occurred in prison hospitals). A review of the literature indicates that various limitations mentioned in relevant studies lie in the unreliability of data (doubts about the validity of official statistics, missing data in archives, missing files on the victims, suicide in juvenile institutions not always recorded separately, etc.). The research emphasizes the importance of improving suicide statistics (recording, clearing up the incidents of deaths that are recorded without specification of cause, etc.) in order to plan and enforce suicide prevention and intervention strategies that seem to "work" in a particular milieu and are not debatable (e.g., the use of "suicide proof" cells).

Greece↗
PubMed2005

Oxidative stress and endometriosis.

BACKGROUND: Little is known about the aetiology of endometriosis; however, in the presence of oxidative stress, reactive oxygen species might increase growth and adhesion of endometrial cells in the peritoneal cavity, leading to endometriosis and infertility. Within a study investigating persistent organic compounds and endometriosis, the authors evaluated the association between oxidative stress and endometriosis. METHODS: Women aged 18-40 years who were undergoing laparoscopy were contacted to participate in the study (n = 100); 84 were eligible and agreed to be interviewed; 78 provided blood specimens. Four markers of oxidative stress and antioxidant status were measured in serum for 61 women. Multiple imputation of missing data was used to generate values for the missing oxidative stress data. RESULTS: Thirty-two women had visually confirmed endometriosis at laparoscopy while 52 did not, including 22 undergoing tubal ligation and 30 with idiopathic infertility. There was a weak association between thiobarbituric acid-reactive substances (nmol/ml) and endometriosis, after adjusting for age, body mass index, current smoking, hormone use in the past 12 months, gravidity, serum vitamin E, serum estradiol, and total serum lipids (beta = 1.18; 95% CI-0.04, 2.39). CONCLUSIONS: These results suggest that oxidative stress might play a role in the development and progression of endometriosis, which should be evaluated in larger studies.

Adolescent↗
PubMed2005

Quality determination and the repair of poor quality spots in array experiments.

BACKGROUND: A common feature of microarray experiments is the occurrence of missing gene expression data. These missing values occur for a variety of reasons, in particular, because of the filtering of poor quality spots and the removal of undefined values when a logarithmic transformation is applied to negative background-corrected intensities. The efficiency and power of an analysis performed can be substantially reduced by having an incomplete matrix of gene intensities. Additionally, most statistical methods require a complete intensity matrix. Furthermore, biases may be introduced into analyses through missing information on some genes. Thus methods for appropriately replacing (imputing) missing data and/or weighting poor quality spots are required. RESULTS: We present a likelihood-based method for imputing missing data or weighting poor quality spots that requires a number of biological or technical replicates. This likelihood-based approach assumes that the data for a given spot arising from each channel of a two-dye (two-channel) cDNA microarray comparison experiment independently come from a three-component mixture distribution--the parameters of which are estimated through use of a constrained E-M algorithm. Posterior probabilities of belonging to each component of the mixture distributions are calculated and used to decide whether imputation is required. These posterior probabilities may also be used to construct quality weights that can down-weight poor quality spots in any analysis performed afterwards. The approach is illustrated using data obtained from an experiment to observe gene expression changes with 24 hr paclitaxel (Taxol) treatment on a human cervical cancer derived cell line (HeLa). CONCLUSION: As the quality of microarray experiments affect downstream processes, it is important to have a reliable and automatic method of identifying poor quality spots and arrays. We propose a method of identifying poor quality spots, and suggest a method of repairing the arrays by either imputation or assigning quality weights to the spots. This repaired data set would be less biased and can be analysed using any of the appropriate statistical methods found in the microarray literature.

Algorithms↗
PubMed1999

Determinants of power-frequency magnetic fields in residences located away from overhead power lines.

The Wertheimer-Leeper wire code, originally developed as a surrogate for magnetic-field exposure, has been associated with childhood leukemia in several epidemiologic investigations. However, these and other studies indicate that most between-residence variability in measured magnetic fields remains unexplained by wire codes. To better understand this remaining variability, engineering and demographic data were examined for 333 underground (UG) and very-low current configuration (VLCC) single-family or duplex residences, selected from a database of nearly 1000 residences specifically because their magnetic fields are most likely affected negligibly by overhead power lines. Using linear regression techniques, four factors predictive of the log-transformed residential field were identified: the square-root of the 24-h average net service drop current (this current is equivalent to the current in the grounding system), the log of the number of service drops on the same secondary serving the residence, residence age (four categories), and area type (rural, suburban, or urban). Complete data on ground current and service drops, the two factors with the strongest individual relationships to measured fields, were available for only half of the residences in the sample. However, these data were determined to be "missing at random" according to established statistical criteria. The full-sample or "composite" models thus relied on a method similar to regression imputation, accounting for missing data with binary dummy variables. When applied to the samples from which they were derived, these models accounted for 25% of the variance of the log-spot-measured magnetic field values in the full sample, while models that considered only those residences with complete data (n = 167) explained about 35%. The model validated well against a sample of 201 ordinary low current configuration (OLCC) homes selected from the same database.

Child↗
PubMed2006

Holding chambers (spacers) versus nebulisers for beta-agonist treatment of acute asthma.

BACKGROUND: In acute asthma inhaled beta2-agonists are often administered to relieve bronchospasm by wet nebulisation, but some have argued that metered-dose inhalers with a holding chamber (spacer) can be equally effective. Nebulisers require a power source and need regular maintenance, and are more expensive in the community setting. OBJECTIVES: To assess the effects of holding chambers (spacers) compared to nebulisers for the delivery of beta2-agonists for acute asthma. SEARCH STRATEGY: We last searched the Cochrane Airways Group trials register in January 2006 and the Cochrane Central Register of Controlled Trials (The Cochrane Library, Issue 4, 2005). SELECTION CRITERIA: Randomised trials in adults and children (from two years of age) with asthma, where spacer beta2-agonist delivery was compared with wet nebulisation. DATA COLLECTION AND ANALYSIS: Two reviewers independently applied study inclusion criteria (one reviewer for the first version of the review), extracted the data and assessed trial quality. Missing data were obtained from the authors or estimated. Results are reported with 95% confidence intervals (CI). MAIN RESULTS: This review has been updated in January 2006 and four new trials have been added. 2066 children and 614 adults are now included in 25 trials from emergency room and community settings. In addition, six trials on in-patients with acute asthma (213 children and 28 adults) have been reviewed. Method of delivery of beta2-agonist did not appear to affect hospital admission rates. In adults, the relative risk of admission for spacer versus nebuliser was 0.97 (95% CI 0.63 to 1.49). The relative risk for children was 0.65 (95% CI: 0.4 to 1.06). In children, length of stay in the emergency department was significantly shorter when the spacer was used, with a mean difference of -0.47 hours (95% CI: -0.58 to -0.37). Length of stay in the emergency department for adults was similar for the two delivery methods. Peak flow and forced expiratory volume were also similar for the two delivery methods. Pulse rate was lower for spacer in children, mean difference -7.6% baseline (95% CI: -9.9 to -5.3% baseline). AUTHORS' CONCLUSIONS: Metered-dose inhalers with spacer produced outcomes that were at least equivalent to nebuliser delivery. Spacers may have some advantages compared to nebulisers for children with acute asthma.

Acute Disease↗
PubMed2002

Antibiotics for trachoma.

BACKGROUND: Trachoma is the world's leading cause of preventable blindness. In 1997 the World Health Organization launched an initiative on trachoma control based on the 'SAFE' strategy (surgery, antibiotics, facial cleanliness and environmental improvement). OBJECTIVES: The aim of this review is to assess the evidence supporting the antibiotic arm of the SAFE strategy by assessing the effects of antibiotics on both active trachoma (primary objective) and on Chlamydia trachomatis infection of the conjunctiva (secondary objective). SEARCH STRATEGY: We searched The Cochrane Controlled Trials Register - CENTRAL/CCTR, which contains the Cochrane Eyes and Vision Group specialised register (Cochrane Library Issue 3, 2001), MEDLINE (1966 to August 2001), and EMBASE (1980 to September 2001). We used the Science Citation Index to look for articles that cited the included studies. We searched the reference lists of identified articles and we contacted authors and experts for details of further relevant studies. SELECTION CRITERIA: We included only randomised trials that satisfied either of two criteria: (a) trials in which topical or oral administration of an antibiotic was compared to placebo or no treatment in people with trachoma, (b) trials in which a topical antibiotic was compared with an oral antibiotic in people with trachoma. A subdivision of particular interest was of trials in which topical tetracycline/chlortetracycline was compared with oral azithromycin, as these are the two World Health Organization recommended treatments. DATA COLLECTION AND ANALYSIS: Two reviewers independently assessed trial quality and extracted data. We contacted investigators for missing data. MAIN RESULTS: We found 15 studies that randomised a total of 8678 participants. For both outcomes (active trachoma and laboratory evidence of infection) the results of the chi-square tests suggested that there was significant statistical heterogeneity among the trials. There was also marked clinical heterogeneity. No summary statistics were calculated and we therefore present a narrative summary of the results. For the comparisons of oral or topical antibiotic against placebo/no treatment, the data are consistent with there being no effect of antibiotics but are suggestive of a lowering of the point prevalence of relative risk of both active disease and laboratory evidence of infection at three and 12 months after treatment. For the comparison of oral against topical antibiotics the results suggest that oral treatment is neither more nor less effective than topical treatment. REVIEWER'S CONCLUSIONS: There is some evidence that antibiotics reduce active trachoma but results are not consistent and cannot be pooled.

Anti-Bacterial Agents↗