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Biomedical subjects

W Horwitz

Publications and source records attributed to W Horwitz.

At least 19 recordsLinked to original sources
PubMed1997

The concept of uncertainty as applied to chemical measurements.

The calculation of uncertainty as recommended for physical measurements cannot be transferred readily to chemical measurements. Physical measurements and chemical measurements have entirely different error patterns that behave differently on replication. Correctable local bias predominates in physical systems and random error is minor; random error predominates in chemical systems and bias is difficult to identify and eradicate. Therefore bias must be monitored by randomizing in the interlaboratory environment, a concept not handled by the conventional ISO treatment of uncertainty.

Chemistry Techniques, Analytical↗
PubMed1994

A multiperspective investigation of social competence in children with sickle cell disease.

Compared the social competence and self-perceptions of 4- to 8-year-old children with sickle cell disease (SCD) to a comparison group of healthy children. Social competence ratings were obtained from multiple perspectives, including parents, teachers, and clinic staff members; children provided ratings of self-perceived acceptance and competence. Children in the SCD group were rated as socially competent as their peers in the comparison group and the normative group. Differences across raters were found, however, with parents in the SCD group giving higher ratings than clinic staff members. No differences were revealed between children in the SCD group and the comparison group in their self-perceptions of competence and acceptance. Implications for future studies addressing the situational specificity of social competence and its relevance to health care of chronically ill children are discussed.

Adaptation, Psychological↗
PubMed1984

Effects of scientific advances on the decision-making process: analytical chemistry.

The analytical chemist has taken the science of identification and quantitation of chemical compounds far beyond the capability of the toxicologist to correlate with the results of animal experiments. In pursuing the vanishing zero, however, the analytical chemist has failed to acknowledge the inherent variability of chemical measurements at very low concentrations that manifest itself not only as discrepant values but as an appreciable fraction of false positive and false negative results and as real negative values (the blank or control is greater than the determination). These are the symptoms of a system not in statistical control. The same type of variable results, evident in biological systems as "biological variability," are then manipulated by statisticians as if they were reproducible measurements. Progress in characterizing biological uncertainty cannot be made until the invisible systematic error of individual laboratories is transformed into random error, amenable to the application of statistical principles. In measurement theory, an examination and correction of systematic error requires knowledge or assignment of a "true value," a concept that does not appear to exist in many biological systems.

Animals↗
PubMed1982

Determination of chloride concentration in cheese: collaborative study.

Three samples of ground Gouda cheese containing 1-2% chloride were analyzed by 7 laboratories by 3 methods: oxidation with KMnO4 and HNO3 followed by a Volhard titration; the same but with filtering off the precipitated AgC1 before back-titration; and the general potentiometric method without ashing or oxidation. The data were analyzed by ISO statistics (ISO-R 5725) and by AOAC statistics (Youden), the major differences being the rejection of different values as outliers and in the statement of the precision parameters. The within-laboratory variability (repeatability) is comparable for all 3 methods; the between-laboratory variability (reproducibility) is comparable for the Volhard method with filtration and the potentiometric methods, but the direct Volhard method is inferior. Because of its generality and simplicity, the potentiometric method has been adopted official first action; the Volhard method with filtration has been reinstated official final action as an alternative.

Cheese↗
PubMed1981

Analytical methods for sulfonamides in foods and feeds. II. Performance characteristics of sulfonamide methods.

Important factors in interpretation of methods for sulfonamides in tissues are value of the blank, use or omission of recovery factors, and precision of the methods. For determining sulfonamide in tissues, no interlaboratory collaborative studies have been performed to provide reproducibility parameters. By assuming comparability with other tissue residue methods at equivalent concentrations, it may be anticipated that the coefficient of variation within-laboratories of the Bratton-Marshall method is about 15% at concentrations of a fraction of a part per million. It is estimated that the limit of reliable measurement of the Bratton-Marshall method is about 0.2 ppm, varying with the individual laboratory. This value is higher than the tolerance it is intended to enforce. Obviously, the method in this case has been stretched beyond its original claimed capabilities. This method also has high blanks and low recoveries. Assignment of sufficient resources to the solution of the problem by regulatory agencies has resulted in methods capable of handling the sulfonamide residue problem at 0.1 ppm.

Animal Feed↗
PubMed1980

Quality assurance in the analysis of foods and trace constituents.

An examination of the results of over 50 interlaboratory collaborative studies conducted by the AOAC on various commodities for numerous analytes shows a relationship between the mean coefficient of variation (CV), expressed as powers of 2, with the mean concentration measured, expressed as powers of 10, independent of the determinative method. Some typical CV values are: drug formulations at a 1% concentration, 2%; sulfonamides in feeds at a 0.01% concentration, 4%; pesticide residues and toxic elements, 10(-6) (1 ppm), 16%; and aflatoxins 10(-8) (10 ppb), 32%. Analytical work at trace levels must be constantly monitored through analysis of reference materials and surrogate samples, and by independent replication.

Aflatoxins↗
PubMed1979

IUPAC gas chromatographic method for determination of fatty acid composition: collaborative study.

An international collaborative study of IUPAC methods II.D.19 and II.D.25 for preparation and GLC analysis of fatty acid methyl esters was begun in 1976. The IUPAC methodology, applicable to animal and vegetable oils and fats and fatty acids from all sources, contains special instructions for preparation and analysis of methyl esters of fatty acids containing 4 or more carbon atoms (analysis of milk fat). Twenty-three collaborators participated in the analysis of 5 known mixtures, 4 vegetable oils, 1 fish oil, and 2 butterfats. Several blind duplicate samples were included. The experimental data were subjected to statistical analysis to examine intra- and interlaboratory variation. Reproducibility and accuracy data for the higher fatty acid (14:0-22:1) mixtures and fish and vegetable oils were satisfactory and were in good agreement with results from an AOCS Smalley Committee check sample program involving analysis of the same samples. Typical coefficients of variation (%) at various concentrations were 15 (2% level), 8.5 (5% level), 7 (10% level), and 3 (50% level). Low recoveries and poor reproducibility were characteristics of results obtained for butyric acid in the butterfat and related known mixtures. A coefficient of variation of about 19% was found for analysis of butyric acid in butterfat, vs coefficients of variation in the range of 4-13% for similar levels of other components in butterfat and other samples. The IUPAC methodology for GLC analysis of fats and oils other than milk fats has been adopted by the AOAC as official first action to replace the current GLC method, 28.063-28.067.

Animals↗
PubMed1977

The variability of AOAC methods of analysis as used in analytical pharmaceutical chemistry.

Pharmaceutical analytical chemistry, which ordinarily deals with the analysis of formulations containing from 0.1 to 100% of active ingredient, uses methods with a reproducibility (between-laboratory variability) of about 2.5% and a repeatability (within-laboratory variability) of about half that amount. The best between-laboratory precision attainable appears to be about 1.0% and within-laboratory precision, about 0.5%. On the basis of the results available, automated methods do not appear to be any more precise than manual methods, although the studies show fewer outlying data points. Replicates (preferably blind ones) should always be conducted in a collaborative interlaboratory study in order to obtain the important information as to whether efforts should be concentrated on improving the method itself or on the performance of laboratories and analysis in applying it.

Chemistry, Pharmaceutical↗
PubMed1976

Problems of sampling and analytical methods.

The need for acceptable, reliable, and practical methods of analysis, chemical, physical, and biological, by the Food Standards Program of the Food and Agriculture Organization/World Health Organization is leading to cooperation by those organizations with the resources and experience to supply them. Methods should be shown to be workable and practicable and then validated in a properly designed international collaborative study for the efficient utilization of the time and effort of participating organizations and laboratories. The methods to be subjected to interlaboratory collaborative study should be clearly written so that the method itself is being tested without unauthorized variations. Satisfactory reference standard materials are often an essential part of the method. Uniform, international methods which have been developed through interlaboratory collaborative studies are applicable to the areas of microbiology and toxicology, as well as chemistry.

Food Analysis↗
PubMed1976

A review of sampling plans and collaboratively studied methods of analysis for aflatoxins.

Aflatoxins are the only food contaminants being monitored routinely on an international scale with methods operating at the order of magnitude of 10 mug/kg. At this concentration level, methods of analysis which can achieve coefficients of variation of 30-40% with recoveries of 70% or greater in interlaboratory collaborative studies can be considered eligible for referee status. In most cases, sample reproducibility is the variable limiting the reliability of methods of analysis. The inherent uncertainty of the identity of chromatographically separated entities requires the application of confirmatory tests to verify that the characteristic measured results from the presence of aflatoxin. The methods are also inoperable without a verification of the identity, purity, and concentration of the reference standards used. Screening methods which reliably eliminate negative samples from further consideration are indispensible for the practical operation of monitoring programs.

Aflatoxins↗
PubMedDate not supplied

International coordination and validation of analytical methods.

Most experimentation dealing with analytical methodology in the physical and biological sciences has been conducted within a single laboratory. Method validation by other laboratories was considered not only unnecessary but also detrimental because, in the words of one commentator, 'the results are too variable'. Within the last two decades, however, largely as a result of the requirements of international environmental and food standards programmes, it has become increasingly apparent that a collaborative interlaboratory study is the only way to estimate the variability characteristics of methods of analysis as performed by the typical population of laboratories using the methods. To obtain a common basis for measuring the statistical characteristics of analytical methods, representatives of almost two dozen international organizations meeting in Geneva, in May 1987, approved by consensus a protocol for the design and interpretation of data from collaborative studies of chemical methods of analysis.

Bias↗
PubMedDate not supplied

Laboratory proficiency testing of aflatoxins in corn and peanuts--a cooperative project between Thailand and the United States.

The objective of this project was to conduct an aflatoxin proficiency test program in government, academia, and industry laboratories in Thailand. Aflatoxin-free corn and peanuts and corn and peanuts naturally contaminated with aflatoxins diluted to approximately 25 micrograms/kg were analyzed. Homogeneity of prepared, naturally contaminated test samples was checked on multiple replicates. The test was conducted according to the ISO/IUPAC/AOAC INTERNATIONAL Harmonized Protocol with z scores indicating laboratory performance. The participants used 3 methods: enzyme-linked immunosorbent assay, thin-layer chromatography, and the minicolumn. Of 19 laboratories that reported results for aflatoxins in naturally contaminated corn, 13 (68%) performed satisfactorily, on the basis of the mean obtained by an expert laboratory, a calculated target value for standard deviation, and the z score. Of 21 laboratories that reported results for aflatoxins in naturally contaminated peanuts, 10 (48%) performed satisfactorily. For aflatoxin-free corn, 6 laboratories reported finding aflatoxins at > or = 10 ng/g, chiefly by the minicolumn method; for aflatoxin-free peanuts, 1 laboratory reported finding aflatoxins at > 10 ng/g. Subsequently, a workshop of lectures and laboratory sessions was conducted to improve performance. A new and simple successive outlier removal procedure applied to the same data removed the same laboratories as did the use of z scores.

Aflatoxins↗