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Biomedical subjects

A Agresti

Publications and source records attributed to A Agresti.

16 recordsLinked to original sources
PubMed1992

Log-linear modelling of pairwise interobserver agreement on a categorical scale.

This article uses log-linear models to describe pairwise agreement among several raters who classify a sample on a subjective categorical scale. The models describe agreement structure simultaneously for second-order marginal tables of a multidimensional cross-classification of ratings. Practical difficulties arise in fitting the models, because models refer to pairwise marginal tables of a very large and sparse table. A standard analysis that treats the marginal tables as independent yields consistent estimates of model parameters, but not of the covariance matrix of the estimates. We estimate the covariance matrix using the jackknife. We apply the models to describe agreement between evaluations made by seven pathologists of carcinoma in situ of the uterine cervix, using a five-level ordinal scale. Previous analyses showed differences among the pathologists in their pairwise levels of agreement, but we observe near homogeneity in the dependence structure of their ratings.

Carcinoma in Situ↗
PubMed1992

Modelling patterns of agreement and disagreement.

This article presents a survey of ways of statistically modelling patterns of observer agreement and disagreement. Main emphasis is placed on modelling inter-observer agreement for categorical responses, both for nominal and ordinal response scales. Models discussed include (1) simple cell-probability models based on Cohen's kappa that focus on beyond-chance agreement, (2) loglinear models for square tables, such as quasi-independence and quasi-symmetry models, (3) latent class models that express the joint distribution between ratings as a mixture of clusters for homogeneous subjects, each cluster having the same 'true' rating, and 4) Rasch models, which decompose subject-by-observer rating distributions using observer and subject main effects. Models can address two distinct components of agreement--strength of association between ratings, and similarity of marginal distributions of the ratings.

Linear Models↗
PubMed1990

Detection of bovine leukaemia virus (BLV) infection by DNA probe technology.

Classical serological methods and Southern blot hybridization for the diagnosis of bovine leukaemia virus (BLV) infection have been compared during the first nine months of life of offspring from BLV serum-negative and serum-positive dams belonging to a Friesian dairy herd in Italy. At birth, 9/13 calves analysed showed serum positivity for anti-gp60 BLV antibodies by agar immunodiffusion and/or by ELISA. However, only two calves were positive for BLV integrated proviruses in their lymphocyte DNA. At six months of age, anti-gp60 BLV antibodies and proviral DNA positivities were simultaneously shown only by the two cattle identified as DNA-positive at birth. This pattern remained constant up to nine months of age. Furthermore, analysis of the molecular characteristics of BLV integrated proviruses, carried out by using, as probes, the almost complete proviral genome (Belgian isolate) or a subclone of the env gene radioactively labelled or chemically modified, revealed that the calves under study were infected by a different isolate (Japanese isolate) and that, in one of the cattle, the majority of integrated proviruses was characterized by deletions probably located in the 5' half of the proviral genome.

Animals↗
PubMed1989

Linkage in human heterochromatin between highly divergent Sau3A repeats and a new family of repeated DNA sequences (HaeIII family).

The hybridization of human DNA with three non-cross-hybridizing monomers (68 bp in length) of the heterochromatic Sau3A family of DNA repeats, indicates the coexistence within a Sau3A-positive genomic block of divergent Sau3A units as well as of unrelated sequences. To gain some insight into the structure of these human heterochromatic DNA regions, three previously cloned Sau3A-positive genomic fragments (with a total length of approximately 1900 base-pairs (bp] were sequenced. The analysis of the sequences showed the presence of clustered Sau3A units with different degrees of divergence and of two DNA regions of approximately 100 bp and 291 bp in length, unrelated to the family of repeats. A consensus sequence derived from the 24 identified Sau3A monomers presents, among highly variable regions, two less variant regions of 8 bp and 10 bp in length, respectively. The Sau3A-unrelated DNA fragment 291 bp in length, used as a probe on genomic DNA digested with a series of restriction enzymes, defines a "new" family of DNA repeats possessing periodicities for HaeIII (HaeIII family). Sau3A and HaeIII repeats display a high degree of linkage in a collection of Sau3A-positive genomic recombinant phages.

Base Sequence↗
PubMed1989

In situ hybridization analysis of interstitial C-heterochromatin in marker chromosomes of two human melanomas.

Two distinct marker chromosomes, presenting with intercalated C- and distamycin A-Dapi-positive regions, were observed in a metastatic and a primary melanoma. To establish the origin of these heterochromatic sequences, we performed in situ hybridization analysis using specific probes for human repetitive DNA. The marker of the primary melanoma, m2, a der 16 chromosome resulting from the translocation of the 1q12-qter segment to band q23 of chromosome 16, showed specific hybridization with Sau3A but not with EcoRI sequences at the interstitial C-band. Thus the origin of this region from the normal chromosome 1 pericentromeric heterochromatin, containing both EcoRI and Sau3A sequences, could be established. On the other hand, the marker of the metastatic melanoma, m1, a der 1 chromosome showing an abnormally banded region inserted between 1q11 and 1q21-qter, failed to give any hybridization signals at the C- and distamycin A-Dapi-positive band when the same EcoRI and Sau3A probes were used. Furthermore, no hybridization was observed using either a probe for SatIII-specific sequences (QP23), mapping to chromosome 9 heterochromatic block, or LS6BB, a ribosomal DNA probe. From these data we speculate that more complex molecular rearrangements may have occurred during the transposition of heterochromatin from its original site to m1. The heterochromatin change found in m1 may be related to advanced stages of malignancy.

Chromosome Banding↗
PubMed1989

A survey of models for repeated ordered categorical response data.

We survey models for analysing repeated observations on an ordered categorical response variable. The models presented are univariate models that permit correlation among repeated measurements. The models describe simultaneously the dependence of marginal response distributions on values of explanatory variables and on the occasion of response. We present models for three transformations of the response distribution: cumulative logits, adjacent-category logits, and the mean for scores assigned to response categories. We discuss three methods for fitting the models: maximum likelihood, weighted least squares, and semi-parametric. Weighted least squares is easily implemented with SAS, as illustrated with a study designed to compare a drug with a placebo for the treatment of insomnia.

Classification↗
PubMed1989

Tutorial on modeling ordered categorical response data.

In the past decade there has been great progress in the development of methodology for analyzing ordered categorical data. Logit and log linear model-building techniques for nominal data have been generalized for use with ordinal data. There are many advantages to using these procedures instead of the Pearson chi-square test of independence to analyze ordered categorical data. These advantages include (a) more complete description of the nature of associations and (b) greater power for detecting population associations. This article introduces logit models for categorical data and shows two ways of adapting them to model ordered categorical data. The models are used to analyze a cross-classification table relating mental impairment and parents' socioeconomic status.

Humans↗
PubMed1987

Chromosomal location by in situ hybridization of the human Sau3A family of DNA repeats.

The Sau3A family is a human, clustered, highly repetitive, GC-rich DNA family. In situ hybridization studies with a plasmid carrying a Sau3A monomer as a probe have shown that Sau3A sequences are preferentially concentrated in the heterochromatic regions of human acrocentric chromosomes (D and G groups, both in pericentromeric regions and in cytological satellites) and in pericentromeric heterochromatin of chromosome 1. The same chromosomal locations were observed by using as probes two recombinant phages which carry Sau3A-positive genomic sectors. The two sectors differ for the relative proportions of monomer and multiples of Sau3A repeats, which show different extents of homology to the cloned monomer, and for the presence, in one of the two, of a small amount of an unrelated repeat (alphoid DNA). The similarity of the results obtained with the three probes suggests that heterogeneous Sau3A repeats share the same chromosomal localizations and that the two analyzed genomic sectors may not contain significant amounts of repetitive DNAs other than the Sau3A family. A comparison between the chromosomal locations of Sau3A and EcoRI families of repeats has confirmed that each family is characterized by specific chromosomal locations and that single heterochromatic regions may contain both.

Centromere↗
PubMed1987

A cytogenetic, phenotypic, and molecular study of an immunoblastic lymphoma with a 14q + translocation.

An uncultured immunoblastic lymphoma, obtained from an untreated patient, was examined from a cytogenetic, immunophenotypic, and molecular viewpoint. The B-cell lineage, immunoglobulin light-chain type, and percentage of neoplastic cells were determined immunologically. Karyotyping showed the presence of a 14q + marker and suggested that the donor chromosome was chromosome 8. Southern-blot analysis of DNA from normal and lymphoma cells, using a molecular probes sequences related to the IGHJ and IGK immunoglobulin genes, confirmed the immunophenotype. A similar analysis, using probes homologous to IGHAC and MYC genes, showed that the t(8;14) detected by cytogenetic analysis resulted in a IGHAC-MYC rearrangement.

Antigens, Surface↗
PubMed1976

The effect of taurine on calcium exchange of sarcoplasmic reticulum of guinea pig heart studied by means of dialysis kinetics.

Calcium binding and release by sarcoplasmic reticulum vesicles of guinea pig heart were studied by means of a dialysis apparatus and analyzed with a model of three closed compartments. The rate constants of calcium binding were increased by taurine, and this effect was dose-related, whereas calcium efflux rate constants were very near to 0 in control experiments and were not effected by taurine. Taurine therefore increased the calcium content of sarcoplasmic vesicles and decreased the calcium concentration of the medium. beta-Alanine had no action on calcium fluxes.

Animals↗
PubMed1976

A computer method for the kinetic analysis of enzyme activity.

A computer method is described in which an estimate of Km and V parameters is obtained by fitting a Michaelis-Menten hyperbola to experimental data. The frequency distribution of calculated parameters is far from normality. A statistical evaluation of the differences between kinetic parameters must be obtained by the use of non-parametric statistics.

Computers↗
PubMedDate not supplied

A new model for ordinal pain data from a pharmaceutical study.

For the pain data analysed previously by Cox and Chuang, this paper proposes a new model that assumes monotone scores for ordered response categories. This proposed model possesses several attractive features and allows a stochastic ordering of the drugs under comparison. Such a model also provides insight regarding the ordinal scale used to classify response. Estimation of the parameters in the model is obtained by use of BMDP3R.

Analgesics↗