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PubMed1999

Applications in survival analysis.

Survival or failure time traits such as herd life and days open are both important economically and pose a number of challenges to an analysis based on linear mixed models. The main features of a survival trait are that it is the time until some event occurs, and some of the observations are censored. Survival models and the associated estimation procedures provide a flexible means of modeling survival traits. In this paper I will discuss the application of survival analysis based on the Weibull distribution. The components that make up a survival model will be presented along with their interpretation. Issues related to the model construction and estimation will be presented.

Animals↗
PubMed2004

Genetic components of days open under heat stress.

A reaction norm approach was used to estimate the genetic parameters of days open (DO) with a model that accounted for heat stress. Data included DO records for Georgia, Tennessee, and North Carolina in the Southeastern United States. A fixed effect model included herd-year, month of calving (MOC), age of cow, and a regression on 305-d milk yield. The reaction norm model additionally included the effect of animal with random regression on a heat stress index (HI), calculated as the standardized solutions to MOC derived from the fixed effect model; the residual variance was assumed to be a function of the HI. The shape of the distribution of the HI was close to a sinusoidal function with the highest value in March/April and the lowest value in September. Genetic and residual variances and heritabilities were highest for spring calvings and lowest for fall calvings. The variance associated with the random regression of the highest level of HI was 33% of the genetic variance of the regular animal genetic effect. Genetic correlation between these effects was 0.67. As a validation, DO data were grouped into 4 seasons of calving and treated as different traits. A 4-trait mixed linear model that included the fixed effects listed above except MOC, was used to analyze the grouped data. In general, the estimates of genetic and residual variances of the multiple trait analyses followed those of the reaction norm model. Genetic correlations of spring with summer, and fall with winter were both 0.90. Genetic correlations between spring/summer and fall/winter were around 0.80. The reaction norm model for DO allows inexpensive genetic evaluation of fertility under heat stress. Results of such an evaluation may strongly depend on editing criteria and model specifications.

Analysis of Variance↗
PubMed2006

Rescuer fatigue: standard versus continuous chest-compression cardiopulmonary resuscitation.

OBJECTIVES: Continuous chest-compression cardiopulmonary resuscitation (CCC-CPR) has been advocated as an alternative to standard CPR (STD-CPR). Studies have shown that CCC-CPR delivers substantially more chest compressions per minute and is easier to remember and perform than STD-CPR. One concern regarding CCC-CPR is that the rescuer may fatigue and be unable to maintain adequate compression rate or depth throughout an average emergency medical services response time. The specific aim of this study was to compare the effects of fatigue on the performance of CCC-CPR and STD-CPR on a manikin model. METHODS: This was a prospective, randomized crossover study involving 53 medical students performing CCC-CPR and STD-CPR on a manikin model. Students were randomized to their initial CPR group and then performed the other type of CPR after a period of at least two days. Students were evaluated on their performance of 9 minutes of CPR for each method. The primary endpoint was the number of adequate chest compressions (at least 38 mm of compression depth) delivered per minute during each of the 9 minutes. The secondary endpoints were total compressions, compression rate, and the number of breaks taken for rest. The students' performance was evaluated on the basis of Skillreporter Resusci Anne (Laerdal, Wappingers Falls, NY) recordings. Primary and secondary endpoints were analyzed by using the generalized linear mixed model for counting data. RESULTS: In the first 2 minutes, participants delivered significantly more adequate compressions per minute with CCC-CPR than STD-CPR, (47 vs. 32, p = 0.004 in the 1st minute and 39 vs. 29, p = 0.04 in the 2nd minute). For minutes 3 through 9, the differences in number of adequate compressions between groups were not significant. Evaluating the 9 minutes of CPR as a whole, there were significantly more adequate compressions in CCC-CPR vs. STD-CPR (p = 0.0003). Although the number of adequate compressions per minute declined over time in both groups, the rate of decline was significantly greater in CCC-CPR compared with STD-CPR (p = 0.0003). The mean number of total compressions delivered in the first minute was significantly greater with CCC-CPR than STD-CPR (105 per minute vs. 58 per minute, p < 0.001) and did not change over 9 minutes in either group. There were no differences in compression rates or number of breaks between groups. CONCLUSIONS: CCC-CPR resulted in more adequate compressions per minute than STD-CPR for the first 2 minutes of CPR. However, the difference diminished after 3 minutes, presumably as a result of greater rescuer fatigue with CCC-CPR. Overall, CCC-CPR resulted in more total compressions per minute than STD-CPR during the entire 9 minutes of resuscitation.

Adult↗
PubMed2003

Transthoracic impedance does not decrease with rapidly repeated countershocks in a swine cardiac arrest model.

STUDY PURPOSE: Successful defibrillation is dependent upon the delivery of adequate electrical current to the myocardium. One of the major determinant of current flow is transthoracic impedance. Prior work has suggested that impedance falls with repeated shocks during sinus rhythm. The purpose of this study was to evaluate changes in transthoracic impedance with repeated defibrillation shocks in an animal model of cardiac arrest due to ventricular fibrillation (VF). METHODS: VF was electrically induced in anesthetized swine. After 5 min of untreated VF, monophasic or biphasic waveform defibrillation was attempted using a standard sequence of 'stacked shocks' (200, 300, then 360 J, if necessary) administered via adhesive electrodes. If one of the first three shocks failed to convert VF, conventional CPR was initiated and defibrillation (360 J) attempted 1 min later. Strength-duration curves for delivered voltage and current were measured during each shock and transthoracic impedance calculated. Animals requiring a minimum of four shocks were selected for study inclusion. Impedance data from sequential shocks were analyzed using mixed linear models to account for the repeated-measures design and the variability of the initial impedance of individual animals. RESULTS: Thirteen animals (monophasic waveform, n=7, biphasic waveform, n=6) required at least four shocks to terminate VF (range 4-6). Transthoracic impedance did not change from the first shock in the 13 animals (46+/-8 Omega) to the fourth shock (46+/-9 Omega). In animals receiving more than four shocks, transthoracic impedance likewise did not change significantly from the first to the last shock, which terminated VF. The lack of a significant change in impedance was also observed when animals were analyzed according to defibrillation waveform. CONCLUSION: Transthoracic impedance does not change significantly with repeated shocks in a VF cardiac arrest model. This is likely due to the lack of reactive skin and soft tissue hyperemia and edema observed in non-arrest models.

Animals↗
PubMed2000

Cerebral blood flow velocity changes after bovine natural surfactant instillation.

OBJECTIVE: To determine the effects of bovine natural surfactant (beractant) instillation on cerebral hemodynamics in preterm infants with respiratory distress syndrome (RDS). STUDY DESIGN: Preterm infants who required surfactant for RDS were enrolled. Cerebral blood flow velocity (CBFV) waveforms from the pericallosal artery were analyzed by pulsed Doppler ultrasonography with the anterior fontanel serving as an acoustic window. CBFV was measured before and at 5, 10, 20, and 30 minutes after the first dose of a bolus instillation of surfactant in four aliquots. Simultaneously with CBFV measurements, mean blood pressure (MBP), heart rate, and ventilator settings were recorded. pH, PACO2, and PAO2 before and at 30 minutes after surfactant administration were also determined. RESULTS: The 30 enrolled preterm infants had a mean birth weight of 973 gm (513 to 1996 gm) and a mean gestational age of 27 weeks (23 to 33 weeks). Mean postnatal age at surfactant administration was 4.7 +/- 2.7 hours. There were no significant changes in pH and PACO2 before and at 30 minutes after surfactant (before surfactant: mean pH of 7.29 +/- 0.07 and mean PACO2 of 44.4 +/- 7.1 torr; after surfactant: mean pH of 7.31 +/- 0.07 and mean PACO2 of 42.7 +/- 8.3 torr). PAO2 increased significantly from a pre-surfactant mean of 83 torr to 130 torr at 30 minutes after surfactant (p < 0.05), with no significant changes in mean airway pressure. There were no significant changes in MBP, heart rate, mean CBFV, peak systolic flow velocity, and diastolic flow velocity before and after surfactant instillation regardless of gestational age. Individual changes in mean CBFV were related to MBP changes (p < 0.001, linear mixed models with random effects). CONCLUSION: In low birth weight infants with RDS, bovine surfactant instillation is not associated with a significant alteration in cerebral hemodynamics. However, the direct relationship between CBFV and MBP is consistent with the reported pressure-passive cerebral circulation in sick preterm infants.

Animals↗
PubMed2005

The effects of different knowledge-dissemination interventions on the mastitis knowledge of Tanzanian smallholder dairy farmers.

We developed three different knowledge-dissemination methods for educating Tanzanian smallholder farmers about mastitis in their dairy cattle. The effectiveness of these methods (and their combinations) was evaluated and quantified using a randomised controlled trial and multilevel statistical modelling. To our knowledge, this is the first study that has used such techniques to evaluate the effectiveness of different knowledge-dissemination interventions for adult learning in developing countries. Five different combinations of knowledge-dissemination method were compared: 'diagrammatic handout' ('HO'), 'village meeting' ('VM'), 'village meeting and video' ('VM+V'), 'village meeting and diagrammatic handout' ('VM+HO') and 'village meeting, video and diagrammatic handout' ('VM+V+HO'). Smallholder dairy farmers were exposed to only one of these interventions, and the effectiveness of each was compared to a control ('C') group, who received no intervention. The mastitis knowledge of each farmer (n=256) was evaluated by questionnaire both pre- and post-dissemination. Generalised linear mixed models were used to evaluate the effectiveness of the different interventions. The outcome variable considered was the probability of volunteering correct responses to mastitis questions post-dissemination, with 'village' and 'farmer' considered as random effects in the model. Results showed that all five interventions, 'HO' (odds ratio (OR)=3.50, 95% confidence intervals (CI)=3.10, 3.96), 'VM+V+HO' (OR=3.34, 95% CI=2.94, 3.78), 'VM+HO' (OR=3.28, 95% CI=2.90, 3.71), 'VM+V' (OR=3.22, 95% CI=2.84, 3.64) and 'VM' (OR=2.61, 95% CI=2.31, 2.95), were significantly (p<0.0001) more effective at disseminating mastitis knowledge than no intervention. In addition, the 'VM' method was less effective at disseminating mastitis knowledge than other interventions. Combinations of methods showed no advantage over the diagrammatic handout alone. Other explanatory variables with significant positive associations on mastitis knowledge included education to secondary school level or higher, and having previously learned about mastitis by reading pamphlets or attendance at an animal-health course.

Animals↗
PubMed1993

Associations of breeding values for disease traits and genetic markers in dairy cattle estimated with a mixed model.

Since 1985, all veterinary treatments on dairy cows in Sweden have been recorded and utilized for estimation of sire breeding values for resistance to clinical mastitis and to diseases other than clinical mastitis. The purpose of the present investigation was to study the relationship of blood groups and blood protein polymorphisms with these traits and to extend a method previously applied to international comparisons of dairy sires for milk production to estimation of marker gene effects. The data comprised 1171 AI sires of the Swedish Red and White dairy breed that had a blood typing record and breeding values for disease resistance. Information was available on the breeding values of sires, the number of daughters of each sire, and the relationships among sires. This information was used to deregress the sire breeding values to average daughter performances, which then were analyzed by a mixed linear model. The reduction in error variance that was due to inclusion of the markers in the model was small. Several substitution effects of the markers were statistically significant. The largest effects corresponded to a deviation of .7 percentage units for the frequency of veterinary treatments among the average daughters of the sires.

Alleles↗
PubMed2006

Improving rescreening in community clinics: does a system approach work?

Community clinics provide inadequate breast cancer screening services to low-income, racially- and ethnically-diverse communities. This study develops and evaluates the effectiveness of multifaceted organizational system interventions--operational assessments, tracking systems, reminder calls, tailored education, physician prompts and a tailored counseling call--on mammography rescreening rates within three community clinics. We used the Chronic Care Model and Put Prevention Into Practice framework to redesign breast screening delivery services within the California Cancer Detection Programs: Every Woman Counts (CDP:EWC), community clinic settings. We used a quasi-experimental design with a random selection of 400 patients at pre-intervention. To establish a post-intervention clinic's rescreening rate a new comparable cross-sectional random sample of 347 women was drawn. Measures A chart abstraction instrument was used to establish clinics' rescreening rates. Subjects participants at pre and post-intervention were low-income women 50 years of age and older who had received normal mammography results and had not been diagnosed with breast cancer in the last five years. General linear mixed model analysis revealed significant improvements for the organizational system redesign condition [pre-intervention rescreening rate: 32.1 percent v. post-intervention rescreening rate 50.2 percent, (p < .001)]. For the organizational system redesign plus tailored counseling call condition, there was maintenance in the rescreening rate following the intervention [pre-intervention: 44.4 percent v. post-intervention: 45.1 percent, (p > 0.05)]. Multilevel interventions directed at redesigning community clinics primary care breast cancer screening services, can improve mammography rescreening rates.

Aged↗
PubMedDate not supplied

An alternate method of radiofrequency neurotomy of the sacroiliac joint: a pilot study of the effect on pain, function, and satisfaction.

BACKGROUND AND OBJECTIVES: The sacroiliac joint (SIJ) can be a source of chronic refractory mechanical spine pain. Few previous studies have described radiofrequency (RF) sensory denervation of the SIJ; results have been inconsistent and technically demanding. This uncontrolled, prospective, cohort study evaluates the effects of an innovative method of RF ablation of the posterior sensory nerves of the SIJ on pain, analgesic use, disability, and satisfaction of patients suffering with chronic mechanical SIJ pain. METHODS: Nine subjects with SIJ pain, confirmed by a local anesthetic joint block, were studied. Subjects were treated with a series of RF strip lesions performed adjacent to the lateral dorsal foraminal aperture plus conventional monopolar lesioning at the L5 dorsal ramus. Each subject completed a questionnaire twice before and at 1, 3, 6, 9, and 12 months after the procedure. The questionnaire evaluated pain intensity and frequency, analgesic intake, disability, satisfaction (with current pain level and the RF procedure), and procedure complications. Data were analyzed by using linear mixed model analysis. RESULTS: After the procedure, significant reductions of back and leg pain frequency and severity, analgesic intake, and dissatisfaction with their current level of pain occurred. Complications were minimal. Overall, 8 of 9 subjects were satisfied with the procedure. CONCLUSIONS: RF sensory ablation of the SIJ using bipolar strip lesions is a technically uncomplicated and low-risk procedure. The resulting effects on pain, disability, and satisfaction are promising. Further evaluation of this technique, including randomized controlled trials, is recommended.

Adult↗
PubMed2004

Risk of wrist fracture in women is heritable and is influenced by genes that are largely independent of those influencing BMD.

UNLABELLED: Using a classical twin design study, we estimated the genetic contribution to liability of wrist fracture in women to be statistically and clinically significant. BMD is highly heritable, but statistical models showed very little overlap of shared genes between the two traits. INTRODUCTION: Studies have observed contradictory evidence for genetic effects influencing the outcome of osteoporotic fracture, in part because of the methodological problems involved in analyzing age-related "censored" outcomes. Although a shared genetic etiology is often assumed between fracture and low BMD, this has not been shown to be the case. MATERIALS AND METHODS: In a study of 6570 white healthy female volunteer twins between 18 and 80 years of age, we identified and validated 220 nontraumatic wrist fracture cases. From this we estimated the population prevalence, case-wise twin concordance, heritability in liability to wrist fracture (WF), and the genetic contribution to WFs controlling for age by analyzing the survival outcome using generalized linear mixed models implemented in Winbugs software. We included forearm BMD as a co-variate in some of the models to test whether there is a shared genetic etiology between WFs and BMD. RESULTS: The prevalence of WFs in women was estimated to be 3.3% with a case-wise concordance in monozygotic twins of 0.28 and 0.11 in dizygotic twins. The additive polygenic heritability in liability was approximately 54%, and a significant genetic etiology was confirmed by analyzing WFs as a survival outcome. The magnitude of the genetic influence on risk of WFs reduced very little when BMD was included as a co-variate in the survival analysis model. CONCLUSIONS: There is an important genetic contribution to the risk of WFs, but for the most part, these genes are unlikely to play a direct etiological role in the development of low BMD. If these results are confirmed for other sites, fracture and low BMD will have their own specific genetic risk factors that are unlikely to be shared between the two traits. This has important clinical and research implications.

Adolescent↗
PubMed1999

Evaluation of interrater reliability for posture observations in a field study.

This paper examines the interrater reliability of a quantitative observational method of assessing non-neutral postures required by work tasks. Two observers independently evaluated 70 jobs in an automotive manufacturing facility, using a procedure that included observations of 18 postures of the upper extremities and back. Interrater reliability was evaluated using percent agreement, kappa, intraclass correlation coefficients and generalized linear mixed modeling. Interrater agreement ranged from 26% for right shoulder elevation to 99 for left wrist flexion, but agreement was at best moderate when using kappa. Percent agreement is an inadequate measure, because it does not account for chance, and can lead to inflated measures of reliability. The use of more appropriate statistical methods may lead to greater insight into sources of variability in reliability and validity studies and may help to develop more effective ergonomic exposure assessment methods. Interrater reliability was acceptable for some of the postural observations in this study.

Analysis of Variance↗
PubMedDate not supplied

Predicting adolescents' intake of fruits and vegetables.

OBJECTIVE: To explore potential predictors of adolescents' fruit and vegetable intake by expanding on current theory and drawing from other adolescent research. DESIGN: This research reports on baseline and interim data from a school-based intervention study. Data were collected through surveys administered to students at the beginning and end of their 7th grade year. SETTING: The students attended 16 public schools in Minnesota. PARTICIPANTS: Data were collected on 3878 students; approximately half were female and 67% were white. All students in the 7th grade cohort were invited to participate in the surveys and over 94% completed both surveys. VARIABLES MEASURED: Our dependent variable, fruit and vegetable intake, was assessed by a validated fruit and vegetable food frequency scale. Predictive factors assessed included parenting style, spirituality/religiosity, depressive symptoms, and other commonly assessed predictors. ANALYSIS: Generalized linear mixed model regression. Omnibus test of association using P <.05 is reported. RESULTS: Subjective norms, barriers, knowledge, usual food choice, parenting style, spirituality/religiosity, and depressive symptoms were statistically significant predictors of intake. The model explained about 31% of the variance in fruit and vegetable consumption. CONCLUSIONS AND IMPLICATIONS: To better understand adolescents' fruit and vegetable intake, we must explore novel predictors. Our results need to be replicated, and more exploratory research in this field is needed.

Adolescent↗
PubMed2006

Myoelectric manifestations of fatigue at low contraction levels in subjects with and without chronic pain.

The aim of the present study was to investigate differences in myoelectric responses to fatigue development between cases with chronic neck-shoulder pain (n=10) and healthy controls (n=10) during a low force level sustained contraction. Subjects performed a 15-min isometric shoulder elevation at a force level of 40 N (sustained contraction), preceded and followed by a step contraction, consisting of five force levels from 20 to 100 N. EMG recordings were made with a two-dimensional electrode array on the upper trapezius of the dominant side. Root-mean-square (RMS(G)), median power frequency (FMED(G)), conduction velocity (CV), number of motor unit action potentials per second (MUAP Rate) and MUAP shape properties were estimated. Changes over time and differences between the groups were statistically evaluated with a linear mixed model. During the sustained contraction, cases showed less increase in RMS(G) than controls (controls: 58.5%, cases: 33.0%). FMED(G) and CV decreased in controls (FMED(G): -6.3%, CV: -5.3%) and stayed constant (FMED(G)) or slightly increased (CV, 3.15%) in cases. Overall, cases showed a less pronounced myoelectric response to the fatiguing task than controls, which may be related to additional recruitment of higher-threshold MUs. A possible explanation might be that cases were already (chronically) fatigued before the experiment started.

Adult↗
PubMed2004

Blind separation of positive sources by globally convergent gradient search.

The instantaneous noise-free linear mixing model in independent component analysis is largely a solved problem under the usual assumption of independent nongaussian sources and full column rank mixing matrix. However, with some prior information on the sources, like positivity, new analysis and perhaps simplified solution methods may yet become possible. In this letter, we consider the task of independent component analysis when the independent sources are known to be nonnegative and well grounded, which means that they have a nonzero pdf in the region of zero. It can be shown that in this case, the solution method is basically very simple: an orthogonal rotation of the whitened observation vector into nonnegative outputs will give a positive permutation of the original sources. We propose a cost function whose minimum coincides with nonnegativity and derive the gradient algorithm under the whitening constraint, under which the separating matrix is orthogonal. We further prove that in the Stiefel manifold of orthogonal matrices, the cost function is a Lyapunov function for the matrix gradient flow, implying global convergence. Thus, this algorithm is guaranteed to find the nonnegative well-grounded independent sources. The analysis is complemented by a numerical simulation, which illustrates the algorithm.

Algorithms↗
PubMed2000

Issues in use of SAS PROC.MIXED to test the significance of treatment effects in controlled clinical trials.

A project that originated with the aim of documenting the implications of dropouts for tests of significance based on general linear mixed model procedures resulted in recognition of problems in the use of SAS PROC.MIXED for this purpose. In responding to suggestions and criticisms, we have further analyzed simulated clinical trial data with realistic autoregressive structure, using alternative error model formulations, different approaches to the use of covariates to model dropout patterns, and different ways to include the critical time variable in the mixed model. Results emphasize the sensitivity of the PROC.MIXED tests of significance for GROUP and TIME x GROUP equal slopes hypothesis to less than optimal modeling of the error covariance structure. Even with the authoritatively recommended best available modeling of the error structure, model formulations that made use of the REPEATED statement did not maintain conservative test sizes when covariates were required to model dropout data patterns. Random coefficients models that employed the RANDOM statement did permit appropriate covariate controls, but the tests of significance for treatment effects were lacking in power. After examining a variety of alternative PROC.MIXED model formulations, it is concluded that none provided both Type I error protection and power comparable to that of simple two-stage analysis of covariance (ANCOVA) procedures for confirming the presence of true treatment effects in controlled clinical trials. Other issues examined in this article concern treating baseline scores as both covariate and initial repeated measurement to which a linear means model is fitted, failure to take advantage of the regression of repeated measurements on time in modeling time as an unordered categorical variable, and fitting linear regression models to nonlinear response patterns.

Algorithms↗
PubMed2000

Between-subject and within-subject statistical information in dental research.

The evaluation of risk factors in dental research frequently uses observations at multiple sites in the same patient. For this reason, statistical methods that accommodate correlated data are generally used to assess the significance of the risk factors (e.g., generalized estimating equations, generalized linear mixed models). In applications of these methods, it is typically assumed (implicitly, if not explicitly) that between-subject and within-subject comparisons will produce the same estimated effect of the risk factor. When between- and within-subject comparisons conflict, the statistical methods can give biased estimates or results that are difficult to interpret. For illustration, we present two examples from periodontal disease studies in which different statistical methods give different estimates and significance levels for a risk factor. Statistical analyses in dental research should assess whether different sources of information give similar conclusions about risk factors or treatments.

Confounding Factors, Epidemiologic↗
PubMed2006

Multicomponent variance estimation for binary traits in family-based studies.

In biometrical genetic analyses of binary traits, the use of family data overcomes some limitations of twin studies, particularly in terms of sample size and types of genetic or environmental factors that can be estimated. However, because of computational problems, recent methods in the application of generalized linear mixed models for family data structure have limited the ability to handle large data sets with general covariates. In this paper, we investigate the use of the hierarchical likelihood approach to the analysis of binary traits from family data. In a simulation study, the method is shown to be highly accurate for the estimation of both the variance components and fixed regression parameters, even for small family sizes. For illustration, we analyze a real data set of familial aggregation of preeclampsia, a pregnancy-induced hypertension. When possible, the analysis is compared with the exact maximum likelihood approach.

Analysis of Variance↗
PubMed2002

A randomized trial of behavioral management for continence with older rural women.

Urinary incontinence (UI) is a commonly underreported and underdiagnosed condition. The purpose of this trial was to implement and evaluate behavioral management for continence (BMC), an intervention to manage symptoms of UI with older rural women in their homes. Participants were randomized into BMC or a control group, and 178 were followed for between 6 and 24 months. The intervention involved self-monitoring, bladder training, and pelvic muscle exercise with biofeedback. The primary outcome variable-severity of urine loss-was evaluated by pad test. Secondary variables were episodes of urine loss, micturition frequency, voiding interval, quality of life, and subjective report of severity. Urine loss severity at baseline evaluation was not significantly different in the two groups. But using the generalized linear mixed model analysis, at the four follow-ups, severity of urine loss, episodes of urine loss, quality of life, and subjective report of severity were significantly different. At 2 years the BMC group UI severity decreased by 61%; the control group severity increased by 184%. Self-monitoring and bladder training accounted for most of the improvement. The results support the use of simple strategies based on bladder diaries before implementing more complex treatments.

Aged↗