SEARCH · PubMed Health

Results for “correlation”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,117 records · Page 62Linked to original sources

PubMed2006

Impact of correlation between pesticide parameters on estimates of environmental exposure.

Monte Carlo techniques are increasingly used in pesticide exposure modelling to evaluate the uncertainty in predictions arising from uncertainty in input parameters and to estimate the confidence that should be assigned to modelling results. The approach typically involves running a deterministic model repeatedly for a large number of input values sampled from statistical distributions. A key decision in setting up a probabilistic analysis is whether there is correlation between any of the inputs to the analysis. Pesticide properties are often the most sensitive in exposure assessment. Analysis of the literature demonstrated that there are examples of both positive and negative correlation between the sorption and degradation behaviour of a pesticide, but that general trends are not apparent at present. The inclusion of even weak correlation between sorption and degradation was found to greatly influence a probabilistic analysis of leaching through soil. Correlation will reduce the predicted extent of leaching for pesticides, and it is recommended to set the correlation to zero unless the experimental data support an alternative assumption (i.e. where the correlation is statistically significant (P <or= 0.05) and experimental artefacts can be excluded).

Environmental Exposure↗

PubMed2004

Analysis of data with multiple sources of correlation in the framework of generalized estimating equations.

This paper is motivated by a study of physical activity participation habits in African American women with three potential sources of correlation among study outcomes, according to method of assessment, timing of measurement, and intensity of physical activity. To adjust for the multiple sources of correlation in this study, we implement an approach based on generalized estimating equations that models association via a patterned correlation matrix. We present a general algorithm that is relatively straightforward to program, an analysis of our physical activity study, and some asymptotic relative efficiency comparisons between correctly specifying the correlation structure vs ignoring two sources of correlation in the analysis of data from this study. The efficiency comparisons demonstrate that correctly modeling the correlation structure can prevent substantial losses in efficiency in estimation of the regression parameter.

Algorithms↗

PubMed2005

Concordance correlation coefficient applied to discrete data.

In any field in which decisions are subject to measurements, interchangeability between the methods used to obtain these measurements is essential. To consider methods as interchangeable, a certain degree of agreement is needed between the measurements they provide. The concordance correlation coefficient is an index that assesses the strength of agreement and it has been widely applied in situations in which measurements are made on a continuous scale. Recently the concordance correlation coefficient has been defined as a specific intraclass correlation coefficient estimated by the variance components of a Normal-Normal mixed linear model. Although this coefficient was defined for the continuous scale case, it may also be used with a discrete scale. In this case the data are often transformed and normalized, and the concordance correlation is applied. This study discusses the expression of the concordance correlation coefficient for discrete Poisson data by means of the Poisson-Normal generalized linear mixed model. The behaviour of the concordance correlation coefficient estimate is assessed by means of a simulation study, in which the estimates were compared using four models: three Normal-Normal mixed models with raw data, log-transformed data and square-root transformed data, and the Poisson-Normal generalized linear mixed model. An example is provided in which two different methods are used to measure CD34+ cells.

Biomedical Research↗

PubMed1992

A note on the effect of the intraclass correlation in the multiple reading procedure with a unanimity rule.

The use of multiple reading procedures to improve the performance of a diagnostic test occurs often in practice. Evaluation of the utility of multiple reading procedures, however, usually ignores the effect of the intraclass correlation. This paper provides a quantitative assessment of this effect in the multiple reading procedure with a unanimity rule with respect to sensitivity, specificity, positive and negative predictive values. We have found that when the disease prevalence is rare or moderate (less than or equal to 0.20), use of the multiple reading procedure with a unanimity rule is effective in increasing the positive predictive value of a single reading procedure for the situation in which the variation of responses among different subjects and the intraclass correlation among repeated tests are small. This is, however, not true for the situation in which the disease is rare and the variation of responses among different subjects is large, even when the intraclass correlation is small or 0. Furthermore, when the disease is rare and the variation of responses among subjects is small, a small or moderate intraclass correlation can substantially decrease the positive predictive value that one calculates under the assumption that the intraclass correlation is equal to 0. In general, when the disease is rare or moderate (less than or equal to 0.20), the intraclass correlation between repeated tests and the variation of responses among subjects have little effect on the negative predictive value.

Data Interpretation, Statistical↗

PubMed2001

A generalized concordance correlation coefficient for continuous and categorical data.

This paper discusses a generalized version of the concordance correlation coefficient for agreement data. The concordance correlation coefficient evaluates the accuracy and precision between two measures, and is based on the expected value of the squared function of distance. We have generalized this coefficient by applying alternative functions of distance to produce more robust versions of the concordance correlation coefficient. In this paper we extend the application of this class of estimators to categorical data as well, and demonstrate similarities to the kappa and weighted kappa statistics. We also introduce a stratified concordance correlation coefficient which adjusts for explanatory factors, and an extended concordance correlation coefficient which measures agreement among more than two responses. With these extensions, the generalized concordance correlation coefficient provides a unifying approach to assessing agreement among two or more measures that are either continuous or categorical in scale.

Computer Simulation↗

PubMed2001

Correlation coefficient estimation involving a left censored laboratory assay variable.

When assessing a correlation between two exposure or biological marker variables, one sometimes encounters the problem of indeterminate values for one of the variables due to an assay detection limit. In this event, investigators often report correlation coefficients computed after removing the pairs involving non-detectable values, or after substituting some small constant for those values. These ad hoc practices can lead to bias in both point and confidence interval estimates of the true correlation coefficient. To address this issue, we consider two parametric techniques for estimating the correlation in the presence of left censoring for one of the variables. The first is a maximum likelihood approach, and the second is an adaptation of multiple imputation motivated primarily by potential benefits in confidence interval coverage. Both of the estimators studied reduce to the standard Pearson's correlation coefficient in the event of no censoring, and hence are valid in cases where this measure would be appropriate for the complete data. We assess these approaches empirically and contrast them with ad hoc methods for estimating the correlation between cervicovaginal human immunodeficiency virus (HIV) viral load measurements and CD4+ lymphocyte counts from HIV positive women enrolled in a clinical trial conducted in Bangkok, Thailand.

Anti-HIV Agents↗

PubMed1992

In vitro embryotoxicity of a series of para-substituted phenols: structure, activity, and correlation with in vivo data.

The embryotoxicity of phenol and twelve para-substituted congeners on mid-gestation rat embryos was evaluated in vitro. Through application of correlative procedures and stepwise regression, equations describing the relationship between physical-chemical properties and various measures of activity were developed. Embryotoxicity was quantified by the log of the reciprocal of the potency estimates for reduction in selected growth parameters and induction of four morphological defects. In general, co-cultured hepatocytes ameliorated embryotoxicity; only phenol-induced embryotoxicity was enhanced by the presence of hepatocytes. In the absence of hepatocytes, measures of growth retardation were positively correlated with molar refractivity of the phenols. With hepatocytes, lipophilicity became important for describing the potential to induce growth deficits. The structural defects had varying correlation patterns in both culture systems. Potencies of these congeners in vitro were also compared to maternal and developmental potencies observed in vivo (Kavlock, Teratology, 41:43-59, '90). Two of the congeners were very toxic in both systems. For the remaining congeners, one maternal toxicity measure was found to be positively correlated with embryotoxicity for growth and development in vitro without hepatocytes. With hepatocytes, a broad spectrum of correlations, both positive and negative, were observed between in vivo developmental toxicity endpoints and in vitro embryotoxicity. Data from preliminary dosimetry studies suggest that phenol congeners may accumulate in embryos exposed in vitro more readily than with in vivo exposure. Potency calculations based on dosimetry information may demonstrate better correlations between data and allow additional relationships between chemical structure and activity to be developed.

Animals↗

PubMed1998

Behavioural correlates of testosterone and seasonal changes of steroids in red-winged blackbirds.

I studied the relationship between behaviour and plasma testosterone level (T) and the seasonal changes in T and plasma corticosterone levels (B) in male red-winged blackbirds, Agelaius phoeniceus. I measured T and B using radioimmunoassay, and on the day after taking blood, I observed each male's behaviour for 60 min. The time that males spent conspicuously perched and the number of songs were positively correlated with T, but the proportion of time spent conspicuously perched and the frequency of song were not correlated with T. The frequency of aggressive encounters, sexual chases, epaulet exposure when singing and flights within the territory were positively correlated with T, suggesting a direct role for circulating testosterone influencing male aggressive behaviour. Both T and B increased early in the breeding season, peaked when the first females were receptive, and decreased through the remainder of the breeding season. Late in the season, the presence of a receptive female caused males to have increased T. The peak in T when the first females were receptive, the positive correlation between aggression and T, and the response to a receptive female with increased T support predictions of the challenge hypothesis. T was positively correlated with B, suggesting a cost to the males of maintaining high T. When a receptive female was present on the male's territory, T was negatively correlated with date. Male red-winged blackbirds in Indiana may respond less to receptive females late in the season when benefits associated with protecting paternity and gaining extra-pair fertilizations decrease.Copyright 1998 The Association for the Study of Animal Behaviour. Copyright 1998 The Association for the Study of Animal Behaviour.

Journal Article↗

PubMed1997

Are subjects' perceptual asymmetries on auditory and visual language tasks correlated?: a meta-analysis.

A number of studies have investigated the correlation between subjects' perceptual asymmetries on auditory and visual language tasks. With few exceptions, these studies have found low, nonsignificant correlations between the two types of perceptual asymmetries. However, variations in subjects' perceptual asymmetries on these tasks reflect not only individual differences in language lateralization but also several extraneous variables (e.g., random errors, individual differences in left-right attentional biases). Thus, low correlations between the two types of asymmetries may reflect "masking" by these extraneous variables rather than a "true" cross-modal dissociation. Consistent with this view, a meta-analysis of prior studies revealed a small, but significant correlation between the two types of asymmetries for left-handers. The correlation was nonsignificant for right-handers, presumably reflecting stronger attenuation caused by limited variability in language lateralization in this group. Possible factors mediating significant correlations between the two types of perceptual asymmetries include certain language processes shared by the two types of tasks, cross-modal consistency in individual differences in lateralization of certain modality-specific language processes, and cross-modal consistency in individual differences in left-right attentional biases.

Attention↗

PubMed1996

A study of biomarkers in cervical carcinoma and clinical correlation of the novel biomarker MN.

The MN protein is a newly described biomarker found to be overexpressed in most cervical carcinomas. This study was an effort to evaluate the prognostic importance of tumor MN expression, HPV status, and the presence of other biomarkers in cervical cancers. Tumor DNA and protein for study were extracted from archived frozen tissue. Tumor tissues and controls were evaluated by Western blot analysis for MN, intestinal alkaline phosphatase (IAP), c-myc, and p53 protein overexpression. Immunohistochemistry was performed for MN quantification and the study of expression patterns in histologic subtypes of cervical cancer. HPV data were obtained by PCR amplification of extracted DNA using consensus and type-specific primers. Clinical data were obtained from the patients' records and from the cancer registry. Clinical and molecular data were correlated by chi2, Fisher's exact test, and logistic regression. The results demonstrate that IAP is not overexpressed in clinical specimens of cervical carcinoma, although in somatic cell hybrid experiments, overexpression of IAP correlates with the malignant state. None of 47 tumors, including those which were HPV negative, overexpressed p53. c-myc protein overexpression occurred in 11 of 52 tumors, most of which contained HPV 16, but this was not significantly different from the tumors as a whole. There was no apparent association between MN protein expression and the overexpression of c-myc protein. MN was overexpressed in all cancers and quantitatively varied with the histologic subtype. Specifically, lower expression of MN correlated with adenosquamous and less-differentiated histology (P < 0.01 for grade 3 tumors). Low expression of MN protein also correlated with HPV negativity (P < 0.05). In stage IB and IIA cancers, low expression of MN was associated with deeper cervical stromal invasion (P < 0.03). Further, low expression of MN correlated with lymph node metastases in small (<3.5 cm) IB and IIA cervical cancers (P < 0.04). These data suggest that MN is emerging as a potentially important new biomarker for cervical carcinoma. The overexpression commonly seen in cervical cancer is possibly associated with loss of a critical tumor suppressor gene located on chromosome 11. Low expression of MN antigen appears to correlate with several adverse prognostic features and further prospective study is warranted.

Adult↗

PubMed1994

Coxsackie B1 virus-induced murine myositis: a correlative study of muscular lesions and serological changes.

We investigated the role of humoral factors in the pathogenesis of Coxsackie B1 virus-induced murine myositis (CB1-myositis). At 2, 4 and 8 weeks after inoculation, serum was studied for circulating immune complexes (CIC) (Raji-cell assay), haemolytic complement activity (CH50 titre) and anticytoplasmic autoantibodies (Western blotting, immunoprecipitation) in relation to degree of mononuclear cell infiltration and muscle fibre necrosis. At 2, 4 and 8 weeks, cell infiltration correlated positively to muscle fibre necrosis. From 2 weeks on, moderate quantities of CIC were found in nearly all CB1-inoculated mice, but without correlation to histological changes. Except for a positive correlation of CH50 titre to muscle necrosis at 4 weeks (r = 0.60; P = 0.02), CH50 titres did not correlate to muscle lesions. Anticytoplasmic and other known autoantibody specificities were absent. In conclusion, first, in CB1-myositis CIC occurred from 2 weeks on, but no correlative evidence was found for their involvement in pathogenesis, neither for that of complement nor for anticytoplasmic autoantibodies. Secondly, cell infiltration correlated positively to muscle necrosis, underscoring the importance of cellular mechanisms. Thus, our data do not support, or conclusively exclude, a role for humoral processes in CB1-myositis.

Animals↗

PubMed1998

Specific correlations between relative synonymous codon usage and protein secondary structure.

We found significant species-specific correlations between the use of two synonymous codons and protein secondary structure units by comparing the three-dimensional structures of human and Escherichia coli proteins with their mRNA sequences. The correlations are not explained by codon-context, expression level, GC/AU content, or positional effects. The E. coli correlation is between Asn AAC and the C-terminal regions of beta-sheet segments; it may result from selection for translational accuracy, suggesting the hypothesis that downstream Asn residues are important for beta-sheet formation. The correlation in human proteins is between Asp GAU and the N termini of alpha-helices; it may be important for eukaryote-specific sequential, cotranslational folding. The kingdom-specific correlations may reflect kingdom-specific differences in translational mechanisms. The correlations may help identify residues that are important for secondary structure formation, be useful in secondary structure prediction algorithms, and have implications for recombinant gene expression.

Bacterial Proteins↗

PubMed2001

A new approach to visualizing spectral density functions and deriving motional correlation time distributions: applications to an alpha-helix-forming peptide and to a well-folded protein.

A new approach to visualizing spectral densities and analyzing NMR relaxation data has been developed. By plotting the spectral density function, J(omega), as F(omega)=2 omega J(omega) on the log-log scale, the distribution of motional correlation times can be easily visualized. F(omega) is calculated from experimental data using a multi-Lorentzian expansion that is insensitive to the number of Lorentzians used and allows contributions from overall tumbling and internal motions to be separated without explicitly determining values for correlation times and their weighting coefficients. To demonstrate the approach, (15)N and (13)C NMR relaxation data have been analyzed for backbone NH and C(alpha)H groups in an alpha-helix-forming peptide 17mer and in a well-folded 138-residue protein, and the functions F(omega) have been calculated and deconvoluted for contributions from overall tumbling and internal motions. Overall tumbling correlation time distribution maxima yield essentially the same overall correlation times obtained using the Lipari-Szabo model and other standard NMR relaxation data analyses. Internal motional correlational times for NH and C(alpha)H bond motions fall in the range from 100 ps to about 1 ns. Slower overall molecular tumbling leads to better separation of internal motional correlation time distributions from those of overall tumbling. The usefulness of the approach rests in its ability to visualize spectral densities and to define and separate frequency distributions for molecular motions.

Magnetic Resonance Spectroscopy↗

PubMed2002

Does the subjective evaluation of medical student surgical knowledge correlate with written and oral exam performance?

BACKGROUND: Medical student performance evaluations have historically contained a significant subjective component. Multiple tools are used to assess fund of knowledge including subjective evaluation by faculty and residents as well as objective evaluations through standardized written and oral exams. We hypothesized that subjective evaluation of medical student knowledge would correlate with objective evaluation through written and oral exams. METHODS: Records of consecutive medical students assigned to the surgery clerkship from January 1999 and March 2001 were reviewed. The core surgical rotation consisted of two 4-week blocks on a private, county, or VA hospital service. Surgical knowledge was assessed subjectively by both faculty (FES) and senior residents (RES) using a 10-point scale with verbal anchors. Objective measures of student surgical knowledge included the National Board shelf exam (WE) and a semistructured oral exam (OE). Data are reported as mean +/- SEM. Spearman rank correlation coefficient (r) was used to assess relationships between groups (r > or = 0.5 --> positive correlation). RESULTS: A total of 354 students were evaluated. The mean FES was 7.8 +/- 0.05 (median = 7.75, range 4.75 to 9.75). The mean RES was 7.7 +/- 0.06 (median = 8.0, range 3.5 to 10.0). There was poor correlation between the subjective perception and objective measures of surgical knowledge (Table 1). Comparison of the FES and RES also showed poor correlation (r = 0.38). CONCLUSIONS: Subjective evaluation of surgical knowledge by faculty and residents correlates poorly with performance measured objectively. These results question whether subjective evaluation of surgical knowledge should be included as part of the evaluation process.

Educational Measurement↗

PubMed1997

Multivariate analysis of correlated selection and kin selection, with an ESS maximization method.

Kin selection coefficients are used in two distinct ways. First, these coefficients measure phenotypic correlations that affect the marginal costs and benefits of behaviors. For example, the phenotypic correlation in sex ratio produced by two females in an isolated patch influences the favoured sex ratio. Second, kin selection coefficients describe genotypic correlations that measure fidelity of transmission. For example, a female values daughters vs. nieces according to genotypic correlations. It is widely known that kin selection coefficients may be interpreted as phenotypic or genotypic correlations in different contexts. However, these different interpretations have never been fully separated, and their different role have not been clearly explained. I provide proofs of a generic analytical approach. The technique automatically separates phenotypic correlations among social partners from genotypic components of transmission. The result is a general method that can be derived from first principles and applied to multivariate problems in social evolution. I emphasize a simple, practical maximization method that can be used to calculate equilibrium conditions for complex social interactions.

Animals↗

PubMed1997

Extinction risk in a temporally correlated fluctuating environment.

Usually extinction risk due to environmental stochasticity is estimated under the assumption of white environmental noise. This holds for a sufficiently short correlation time tauc of the fluctuations compared to the internal time scale of population growth r-1 (tauc/r-1<<1). Using a time-discrete simulation model we investigate when the white noise approximation is misleading. Environmental fluctuations are related to fluctuations of the birth and death rates of the species and the temporal correlation of these fluctuations (coloured noise) is described by a first-order autoregressive process. We found that extinction risk increases rapidly with correlation time tauc if the strength of noise is large. In this case the white noise approximation underestimates extinction risk essentially unless temporal correlation is very small (tauc/r-1<<0.1). Extinction risk increases only slowly with correlation time if the strength of noise is small. Then the white noise approximation may be used even for stronger temporal correlations (tauc/r-1>/=0.1). Thus, the estimation of extinction risk on the basis of white or coloured noise must be justified by time scale and strength of the fluctuations. Especially for species that are sensitive to environmental fluctuations the applicability of the white noise approximation should be carefully checked.

Animals↗

PubMed1996

Spinal connections of ventral-group bulbospinal inspiratory neurons studied with cross-correlation in the decerebrate rat.

We examined the synaptic connections from ventral-group bulbospinal inspiratory neurons to upper-cervical inspiratory neurons and phrenic and intercostal motoneurons in decerebrate rats using cross-correlation. Inspiratory neurons were recorded in the medulla (n = 28) at the level of the obex and from the upper-cervical segments (C1 and C2) of the spinal cord (n = 29) in 18 vagotomized, paralyzed, ventilated, and decerebrated rats. The neurons were identified by their inspiratory firing pattern and antidromic activation from the spinal cord at C7. Whole-nerve recordings were made using bipolar electrodes from the central cut ends of the C5 phrenic nerve and the external and internal intercostal nerves at various thoracic levels. Cross-correlation histograms were computed between these recordings to detect short time scale synchronizations indicative of synaptic connections. Cross-correlation histograms (n = 20), computed between the activities of ventral-group bulbospinal inspiratory neurons and the phrenic nerve, all showed peaks (mean half-amplitude width +/- SD, 1.1 +/- 0.3 ms) at short latencies (mean latency +/- SD, 2.0 +/- 0.6 ms) suggestive of monosynaptic excitation. Cross-correlation histograms (n = 33), computed between the activities of ventral-group bulbospinal inspiratory neurons and upper-cervical inspiratory neurons, displayed four (12%) peaks (mean half-amplitude width +/- SD, 0.9 +/- 0.1 ms) at short latencies (mean latency +/- SD, 1.8 +/- 0.6 ms) suggestive of monosynaptic excitation, and six (18%) peaks (mean half-amplitude width +/- SD, 1.4 +/- 0.4 ms) at latencies near zero suggestive of excitation from a common source. Cross-correlation histograms (n = 34), computed between the activities of ventral-group bulbospinal inspiratory neurons and the internal and external intercostal nerves at various thoracic levels (T2-8), showed six (18%) peaks (mean half-amplitude width +/- SD, 2.5 +/- 0.5 ms) at short latency (mean latency +/- SD, 4.5 +/- 1.1 ms) suggestive of oligosynaptic connections. Cross-correlation histograms (n = 42) computed between activities of intercostal nerves at various levels of the thoracic spinal cord showed central peaks suggestive of excitation from a common source. Although the size of the peaks decreased with segmental separation, the displacement of the peaks from time zero did not increase with segmental separation (mean displacement +/- SD, 0.6 +/- 0.6 ms) as would be expected if the common excitation resulted from a descending monosynaptic excitation by a source such as the ventral-group bulbospinal inspiratory neurons. We conclude that all ventral-group bulbospinal inspiratory neurons make monosynaptic connections to phrenic motoneurons, a few make monosynaptic connections to upper-cervical, inspiratory neurons, but connections to intercostal motoneurons are made via interneurons.

Animals↗

PubMed1996

Connections from upper cervical inspiratory neurons to phrenic and intercostal motoneurons studied with cross-correlation in the decerebrate rat.

We examined the synaptic connections from upper cervical inspiratory neurons to phrenic and intercostal motoneurons in decerebrate rats using cross-correlation. Upper cervical inspiratory neurons (n = 79) were recorded from the C1 and C2 segments of the spinal cord in 38 vagotomized, paralyzed, ventilated, and decerebrate rats. The neurons were identified by their inspiratory firing pattern and antidromic activation from the ipsilateral spinal cord at C7. Whole-nerve recordings were made using bipolar electrodes from the central cut ends of the C5 phrenic nerve and the external and internal intercostal nerves at various thoracic levels. Cross-correlation histograms were computed between these recordings to detect short time-scale synchronizations indicative of synaptic connections. The 55 cross-correlation histograms computed between the upper cervical inspiratory neurons and the ipsilateral phrenic nerve showed seven (13%) narrow peaks (mean half-amplitude width +/- SD, 1.09 +/- 0.15 ms) at short latencies (mean latency +/- SD, 1.29 +/- 0.26 ms) suggestive of monosynaptic excitation, and four (7%) broader peaks (mean half-amplitude width +/- SD, 1.50 +/- 0.17 ms) at short latencies (mean latency +/- SD, 1.40 +/- 0.24 ms) suggestive of oligosynaptic excitation. Another 14 (25%) cross-correlation histograms displayed a central broad peak (mean half-amplitude width +/- SD, 1.59 +/- 0.23 ms) suggestive of common activation. The eight cross-correlation histograms computed between the upper cervical inspiratory neurons and the contralateral phrenic nerve were featureless. The 77 cross-correlation histograms computed between the upper cervical inspiratory neurons and the internal and external intercostal nerves at various thoracic levels (T2-8) showed no peaks suggestive of synaptic connections. We conclude that some upper cervical inspiratory neurons make monosynaptic and paucisynaptic connections to phrenic motoneurons but not to intercostal motoneurons.

Animals↗