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PubMed2001

Model selection in estimating equations.

Model selection is a necessary step in many practical regression analyses. But for methods based on estimating equations, such as the quasi-likelihood and generalized estimating equation (GEE) approaches, there seem to be few well-studied model selection techniques. In this article, we propose a new model selection criterion that minimizes the expected predictive bias (EPB) of estimating equations. A bootstrap smoothed cross-validation (BCV) estimate of EPB is presented and its performance is assessed via simulation for overdispersed generalized linear models. For illustration, the method is applied to a real data set taken from a study of the development of ewe embryos.

Bias↗
PubMed2001

Inference procedures for assessing interobserver agreement among multiple raters.

We propose a new procedure for constructing inferences about a measure of interobserver agreement in studies involving a binary outcome and multiple raters. The proposed procedure, based on a chi-square goodness-of-fit test as applied to the correlated binomial model (Bahadur, 1961, in Studies in Item Analysis and Prediction, 158-176), is an extension of the goodness-of-fit procedure developed by Donner and Eliasziw (1992, Statistics in Medicine 11, 1511-1519) for the case of two raters. The new procedure is shown to provide confidence-interval coverage levels that are close to nominal over a wide range of parameter combinations. The procedure also provides a sample-size formula that may be used to determine the required number of subjects and raters for such studies.

Binomial Distribution↗
PubMed2005

Bayesian inferences on predictors of conception probabilities.

Reproductive scientists and couples attempting pregnancy are interested in identifying predictors of the day-specific probabilities of conception in relation to the timing of a single intercourse act. Because most menstrual cycles have multiple days of intercourse, the occurrence of conception represents the aggregation across Bernoulli trials for each intercourse day. Because of this data structure and dependency among the multiple cycles from a woman, implementing analyses has proven challenging. This article proposes a Bayesian approach based on a generalization of the Barrett and Marshall model to incorporate a woman-specific frailty and day-specific covariates. The model results in a simple closed form expression for the marginal probability of conception, and has an auxiliary variables formulation that facilitates efficient posterior computation. Although motivated by fecundability studies, the approach can be used for efficient variable selection and model averaging in general applications with categorical or discrete event time data.

Adult↗
PubMed2006

Co-occurrence of East and West African kdr mutations suggests high levels of resistance to pyrethroid insecticides in Anopheles gambiae from Libreville, Gabon.

Point mutations in the voltage-gated sodium channel gene involved in knockdown resistance to DDT and pyrethroid insecticides have been described in several insect species. In the malaria vector Anopheles gambiae Giles sensu stricto (Diptera: Culicidae) two mutations have been identified. The first, consisting of a leucine-phenylalanine substitution at amino acid position 1014, is widespread in West Africa. The second, a leucine-serine substitution at the same position, has to date only been detected in western Kenya. Analysis of the kdr polymorphism in a sample of 106 An. gambiae s.s. of the rDNA S-form/Type I collected in Libreville (Gabon) surprisingly revealed the presence of both East and West African kdr mutations with frequencies of 63% and 37%, respectively. No wild-type alleles were detected and there was an excess of heterozygous genotypes (P = 0.04). In addition, an inconsistency was found during the kdr genotyping procedures by polymerase chain reaction, which could have lead to an underestimation of resistance alleles. The implications of these findings are discussed.

Animals↗
PubMed1993

Applicability of the critical difference.

The present study is concerned with the critical difference, which may help to judge whether or not the difference between two consecutive measurements with a certain probability (i.e. 95 percent) may be ascribed to natural variation. Knowledge of the applicability of the critical difference in veterinary medicine is sparse and therefore, to justify future use of the critical difference, it is important to know whether or not the critical difference performs as expected. The hypothesis to be tested in this study was that at least 95 percent of the differences between consecutive measurements, that have been obtained in animals where the component measured is known to be unchanged, should be within the critical difference. From previous studies it was known that a low-sodium diet had no influence on the plasma potassium concentration. The critical difference of the plasma potassium concentration in dogs was calculated as 0.5 mmol/l. using weekly measurements of this plasma component in a group of twenty healthy dogs. To test the hypothesis, this value was compared to the differences between consecutive weekly measurements of this plasma component in another group of eight dogs fed a low-sodium diet for five weeks. In agreement with previous studies, the plasma potassium concentration in the eight dogs did not change significantly during the feeding experiment. Of the fourty differences between consecutive weekly measurements, thirty-six were within the critical difference. This number was not different from the number expected from the hypothesis and thus, the critical difference performed as expected.(ABSTRACT TRUNCATED AT 250 WORDS)

Animal Feed↗
PubMed1997

Response to fluconazole and itraconazole of Candida spp. in denture stomatitis.

The significance of Candida albicans in the development of denture stomatitis (DS), as well as the clinical and microbiological efficacy of treatment with fluconazole and itraconazole was studied in 115 patients affected with DS and 200 controls (100 healthy patients with dental prosthesis and 100 healthy patients without prosthesis). Specimens were taken from all patients; subsequently all patients with positive culture of the DS group were treated with fluconazole. A second specimen was taken after 15 days of treatment with fluconazole, and if the results were positive again, treatment with itraconazole was instituted and the patients were given appointments for taking a third specimen. The incidence of C. albicans was 92% in the group of patients with DS. After treatment with fluconazole, a clinical cure of 97% and a microbiological cure of 78% was obtained in the patients with DS. In 3.2% of the cases strains resistant to fluconazole were found. The cases of microbiological resistance to fluconazole were treated with itraconazole resulting in a clinical cure of 100% and a microbiological cure of 77%. The results show the poor correlation of the clinico-microbiological response after treatment with these antifungal agents in denture stomatitis.

Binomial Distribution↗
PubMedDate not supplied

Detecting systematic bias between two raters.

1. I recently reviewed, inter alia, methods for comparing two raters who make judgements on an ordered categorical scale, directed principally at the kappa statistic and its weaknesses. 2. The main weakness of the kappa statistic is that it fails to detect the all-important feature of systematic bias between raters. 3. I described various methods for detecting bias between two raters. These included a modified McNemar test and the single binomial test. Others that have been suggested are the symmetry of disagreement index (SD) and the marginal homogeneity test. 4. I now realize that none of the above four tests for bias is satisfactory, because all ignore the extent of agreement. 5. The bias index (BI) does take into account the extent of agreement, but its inventors did not propose how BI could be evaluated. I now describe a method for doing this.

Algorithms↗
PubMed1995

The association between El Niño/Southern Oscillation events and typhoons in the Marshall Islands.

An analysis of the historic record of typhoons in the Marshall Islands has identified a significant association between the occurrence of the El Niño/Southern Oscillation phenomenon (ENSO) and the occurrence of typhoons in the Marshall Islands. Whilst typhoons normally occur further to the east, the warming of the ocean waters around the Marshall Islands, as part of the ENSO phenomenon, generates typhoons further to the west. The results suggest that typhoons are 2.6 times more likely to occur during ENSO years, with a 71 per cent chance of a typhoon striking during an ENSO year, and only a 26 per cent chance of one happening during a non-ENSO year. This has implications for planning and public safety, which the relevant authorities may wish to take note of.

Atmosphere↗
PubMed1998

Mental health services in Perth nursing homes.

Directors of nursing (DONs) in 42 nursing homes in Perth were asked for information concerning mental health services provided for their residents. A questionnaire similar to those used by other researchers in Sydney and in Ontario was utilised to enable meaningful comparisons among the three studies. According to the DONs' perceptions, the mean proportion of residents in Perth nursing homes with psychiatric and/or behavioural problems was between 50% and 75%. The mental health services provided to nursing homes were significantly less than that desired by the DONs. Other variables assessed in this study included the number of transfers per year because of psychiatric problems; the number of staff with psychiatric qualifications; the provision of psychiatric training for staff; the most frequently occurring psychiatric problems in residents and the most valued mental health services provided to nursing homes. Results were comparable to those of the Sydney and Ontario studies. This study suggests that there is a significant psychiatric population in Perth nursing homes that, like those in Sydney and Ontario are seriously neglected regarding appropriate professional psychiatric care. Existing psychogeriatric community assessment teams could provide more of these services if adequately staffed and resourced.

Attitude of Health Personnel↗
PubMed1980

The sex ratios of anencephalics born to anencephalic-prone women.

Anencephalic infants born to women who are very prone to bear them do not have a sex ratio appreciably different from that of other anencephalic births. Moreover, the sexes of affected cases in sibships containing exactly two anencephalic births seem to be distributed approximately binomially. Bearing in mind the correlation between prevalence at birth and sex ratio in anencephaly, these findings suggest that an environmental teratogen attacks the fetuses of women who are already (perhaps genetically) disposed to bear anencephalic infants.

Anencephaly↗
PubMed1991

Ponderal index: a better definition of the 'at risk' group with intrauterine growth problems than birth-weight for gestational age in term infants.

A study was undertaken at Nepean Hospital, Penrith to compare the outcome of pregnancies with growth retarded infants; 2,508 consecutive births were analyzed between August 1, 1989 and April 30, 1990. A comparison of outcome was made between infants whose birth-weight for gestational age was below the tenth percentile and infants who had a low ponderal index from 37 weeks' gestation. The Caesarean section delivery and fetal distress rates were significantly higher for infants with a low ponderal index than for infants with a low birth-weight. Ponderal index appears to be a better measure of infants with intrauterine growth problems than birth-weight percentiles.

Binomial Distribution↗
PubMed2005

Frequentist performance of Bayesian confidence intervals for comparing proportions in 2 x 2 contingency tables.

This article investigates the performance, in a frequentist sense, of Bayesian confidence intervals (CIs) for the difference of proportions, relative risk, and odds ratio in 2 x 2 contingency tables. We consider beta priors, logit-normal priors, and related correlated priors for the two binomial parameters. The goal was to analyze whether certain settings for prior parameters tend to provide good coverage performance regardless of the true association parameter values. For the relative risk and odds ratio, we recommend tail intervals over highest posterior density (HPD) intervals, for invariance reasons. To protect against potentially very poor coverage probabilities when the effect is large, it is best to use a diffuse prior, and we recommend the Jeffreys prior. Otherwise, with relatively small samples, Bayesian CIs using more informative (even uniform) priors tend to have poorer performance than the frequentist CIs based on inverting score tests, which perform uniformly quite well for these parameters.

Bayes Theorem↗
PubMed1994

The correlated binomial model in the analysis of clinical diagnostic test parameters for recurrence of periodontitis.

In the present study, recall data of 29 patients with advanced periodontitis were utilized to calculate diagnostic test parameters for bleeding scores or increased residual probing depth on increase of periodontal probing depth of > or = 2 mm after 2 years, comparing the simple binomial and correlated binomial model. 5.6% of sites were identified, where the periodontal probing depth increased 2 mm or more during 2 years of maintenance. Within-patient correlation coefficient of sites were found to be small, in general (p < 0.02), with the exceptions for predictive value of a positive test for frequent (> or = 3/4 times) bleeding on probing and residual probing depth of > or = 4 mm (p = 0.137 and p = 0.086, respectively), and specificity for no bleeding (p = 0.151). Point estimates of test parameters generally did not differ between simple binomial and correlated binomial model. The respective standard deviations were either over- or underestimated in the simple binomial model. It was concluded that the suggestion of a limited diagnostic value of bleeding scores or increased residual postoperative probing depth to predict recurrence of periodontitis cannot be rejected after application of a correlated binomial model.

Adolescent↗
PubMed1993

Oral health status of a long-term-care, veteran population.

The Department of Veterans Affairs (VA) operates the largest, integrated health care system in the United States of America. The projected need for long-term-care in the VA health care system parallels an expected increase in need for care in the United States, but precedes the need for care in the general population by 25-30 yr. The VA's Office of Dentistry, in an effort to estimate the resource requirements of this swelling group of veterans, initiated in 1986-7 an oral health survey of long-term care patients. The overall goals were to describe the oral health status of VA nursing home care units (NHCU) residents and to develop a methodology for estimating future dental health services utilization. This study describes the oral health status of the study population. Demographic and oral health data were collected for 650 long-term care residents of six VA NHCUs between October 1986 and July 1987. Data were collected on sociodemographic status, medical history, dental caries, periodontal diseases, oral soft tissue pathology, and the presence of dental prostheses. Caries and periodontal disease were evaluated using the United States National Institute of Dental Research Survey of Employed Adults and Seniors protocols. The oral health status of the population is described using DMF and ESI indices, the prevalence of oral lesions, levels of tooth loss, oral hygiene scores, and the status of existing dentures. Findings show moderate levels of untreated dental caries and periodontal disease and significant tooth loss which increased with age. A need for preventive therapy, restorative dentistry, conservative periodontal therapy, and prosthodontic care was evident.(ABSTRACT TRUNCATED AT 250 WORDS)

Aged↗
PubMed2004

The utility of the zero-inflated Poisson and zero-inflated negative binomial models: a case study of cross-sectional and longitudinal DMF data examining the effect of socio-economic status.

OBJECTIVES: To examine the utility of the zero-inflated Poisson (ZIP) and zero-inflated negative binomial (ZINB) modelling approaches for modelling four sets of dental caries data from the same cohort study [with particular attention to the influence of childhood socioeconomic status (SES)]: cross-sectional data on the deciduous dentition at age 5 years; cross-sectional data on the permanent dentition at age 18 and 26 years; and longitudinal data on caries increment between ages 18 and 26 years. METHODS: Data on dental caries occurrence at ages 5, 18 and 26 years were obtained from the Dunedin Multidisciplinary Health and Development Study (DMHDS). ZIP and ZINB models were fitted to the cross-sectional (n = 745) and longitudinal (n = 809) data sets using Stata (Intercooled Stata 7.0). The dependent variables for the three cross-sectional analyses were the DMFS indices at age 5, 18, and 26 years, and net DFS increment (NETDFS) was the dependent variable for the longitudinal analysis. RESULTS: The empty ZIP model was a poor fit for all four data sets, whereas the empty ZINB model showed good fit; consequently both the cross-sectional and longitudinal analyses were conducted using ZINB modelling. Being in the high-SES group during childhood was associated with a greater probability of being caries-free by age 18 years, over and above that which would be expected from the negative binomial process. Low childhood SES also had the largest coefficient in the modelling of the negative binomial process, but at age 5 years, where the adjusted mean dmfs score in the low-SES group was 6.8 (compared with 4.7 and 2.9 in the medium- and high-SES groups, respectively). The substantial SES differences which existed at age 5 years (in the deciduous dentition) had reduced somewhat by age 18 years, and had widened again by age 26 years. In the longitudinal analysis, "baseline" caries experience (age 18-year DMFS) was a predictor both of being an extra zero and of caries severity. CONCLUSION: This investigation of the utility of the zero-inflated approach for modelling both cross-sectional and longitudinal caries data has shown that ZIP/ZINB models can provide new insight into disease patterns. It is anticipated that they will become increasingly useful in epidemiological studies that use the DMF index as the outcome measure.

Adolescent↗