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PubMed1978

The importance of beta, the type II error and sample size in the design and interpretation of the randomized control trial. Survey of 71 "negative" trials.

Seventy-one "negative" randomized control trials were re-examined to determine if the investigators had studied large enough samples to give a high probability (greater than 0.90) of detecting a 25 per cent and 50 per cent therapeutic improvement in the response. Sixty-seven of the trials had a greater than 10 per cent risk of missing a true 25 per cent therapeutic improvement, and with the same risk, 50 of the trials could have missed a 50 per cent improvement. Estimates of 90 per cent confidence intervals for the true improvement in each trial showed that in 57 of these "negative" trials, a potential 25 per cent improvement was possible, and 34 of the trials showed a potential 50 per cent improvement. Many of the therapies labeled as "no different from control" in trials using inadequate samples have not received a fair test. Concern for the probability of missing an important therapeutic improvement because of small sample sizes deserves more attention in the planning of clinical trials.

Clinical Trials as Topic↗
PubMed1990

Rapid immunotyping of B-cell non-Hodgkin's lymphomas by flow cytometry. A comparison with the standard frozen-section method.

The authors compared immunotyping (IT) results obtained by both standard frozen section (FS) and flow cytometry (FC) methods on 218 biopsies suggestive of lymphoma to learn the advantages of each method. The independent interpretations of the FS and FC IT results were concordant in 93% (202 of 218) of cases. The 16 cases with discordance were reviewed and seven causes for discrepancy found: methodologic problems, focal lymphomatous involvement, more sensitive light chain detection by FC, inadequate sample for FC, interpretation error, sample mislabeling for FC, and unexplained. Eleven of the concordant B-cell non-Hodgkin's lymphomas (NHLs) studied by FC did not have a kappa:lambda ratio of 3 or greater or 0.5 or less and were shown to express light chain restriction by a D-value of 15 or greater with the use of statistical analysis of the kappa and lambda histograms or by multiparameter analysis of large versus small cells. The authors found both methods to be effective for phenotyping lymphomas, however, each has distinct features, making them complementary in their applications.

Antibodies, Monoclonal↗
PubMed1996

[Cervical cancer in Frederiksborg County 1990-1991].

One hundred and eight patients from Frederiksborg County, Denmark with cervical cancer diagnosed from 1990 to 1993 were analysed concerning type of carcinoma, tumour stage and screening history. The following types of carcinoma were found: 87 (81%) squamous, 5 (5%) adenosquamous, 15 (14%) adenocarcinoma and one (1%) small cell carcinoma. All women aged 23-60 receive a written invitation to participate in the screening programme. Of the 57 patients who had never or only sporadically been screened 23 were outside the target population. Tumour stage was generally higher for the non-screened, i.e. only 57.9% stage I compared to 82.4% for the screened population. In 51 cases the following errors had occurred: seven sampling errors, 21 screening errors, 15 lack of follow-up of abnormal or inadequate smears, six inadequate cryotherapy and two interval cancers.

Adenocarcinoma↗
PubMed1997

Comparing measurements of biomarkers with other measurements of exposure.

The issue of the relative merit of biomarkers and alternative measures of exposure arises most commonly in the context of epidemiological studies aimed at hazard detection and quantification. When exposures are from biological agents, biomarkers are usually the first and often the only justifiable choice. In general, however, the relative merit of different types of exposure measurements need to be evaluated on a case-by-case basis. Biomarkers may be affected by random errors, time-related sampling errors, physiological confounding and disease-induced differential error, all of which need to be explicitly evaluated before embarking on the use of a biomarker in a full-scale epidemiological study. Random errors affecting biomarkers may be reduced by replication or combination of measurements, or both. Alternative measurements of exposure can be evaluated against a biomarker when there is adequate evidence for regarding the marker as the true measure of a biologically relevant exposure.

Biomarkers, Tumor↗
PubMed1976

A propagation of error analysis of the enzyme activity expression. A model for determining the total system random error of a kinetic enzyme analyzer.

We present a total system error evaluation of random error, based on a propagation of error analysis of the expression for the calculation of enzyme activity. A simple expression is derived that contains terms for photometric error, timing uncertainty, temperature-control error, sample and reagent volume errors, and pathlength error. This error expression was developed in general to provide a simple means of evaluating the magnitude of random error in an analytical system and in particular to provide an error evaluation protocol for the assessment of the error components in a prototype Miniature Centrifugal Analyzer system. Individual system components of error are measured. These measured error components are combined in the error expressiion to predict performance. Enzyme activity measurements are made to correlate with the projected error data. In conclusion, it is demonstrated that this is one method for permitting the clinical chemist and the instrument manufacturer to establish reasonable error limits.

Aspartate Aminotransferases↗
PubMed1997

Pitfalls in epidemiological analysis.

Epidemiologists rely heavily on the relative risk in their analyses and presentations. As an index it is intelligible and intuitively appealing but can give an exaggerated impression of the strength of the association and is unreliable for comparisons. This can be shown by deriving relative risks from a normal correlation surface with an unimpressive level of correlation. Relative risks ought to be handled with caution; the underlying population risk and the relative size of exposed and reference categories should be reported. Efforts to control for additional variables, confounders, by some kind of multi-variate technique, another standard procedure, could easily give a false sense of security. From time to time it has been made clear in the literature that errors of measurement in the third variable or in the additional variables could lead to the appearance of false independent effects, but these warnings do not seem to have been heeded nearly as much as they deserve. A simulation experiment is used to bring the lesson home, with realistic numerical assumptions. A moderate degree of error contamination will produce spurious effects. This has nothing to do with sampling errors, large samples rather aggravate this danger. In meta-studies this is a source of error and conflicting results to take account of.

Confounding Factors, Epidemiologic↗
PubMed1994

[Quality control of intraoperative diagnosis. Annual review of 1490 frozen sections].

A retrospective evaluation of the accuracy of frozen section diagnoses over 1-year period has been carried out. Of 20998 surgical pathology specimens accessioned during 1991, 1265 (6.0%) had intraoperative pathologic consultation; 1490 were the frozen section examinations effectively carried out, with an average of 1.2 frozen section diagnoses per case. Of all frozen sections performed, 4.6% were deferred; in the remaining cases, the concordance between frozen sections and the final histologic diagnoses was 97%. The reasons for diagnostic discordances were gross sampling error (51.2%), misinterpretation (44.2%), microscopic sampling (9.3%), technical problems in sectioning (2.3%). We believe that a similar continuous monitoring should be performed in every pathology department, to recognize the reasons of errors and, if possible, to reduce them.

Academies and Institutes↗
PubMed1986

Parameters that bias the measurement of airborne concentration within a respirator.

This paper describes the theoretical basis upon which a test system has been set up to evaluate the sampling error associated with in-facepiece sampling on half-mask respirators. The in-facepiece sampling technique evaluated in this study is the one currently used in the U.S. to conduct quantitative facepiece fit testing. An experimental design was developed to study the sampling bias associated with in-facepiece sampling when selected parameters of the man/respirator system were varied. The results indicated that significant errors can be made in estimating concentration within a respirator when the current in-facepiece sampling technique is employed. Sampling bias was determined when in-facepiece samples were collected only during the inhalation phase of the respiratory cycles. They were found to range from greater than -99% to greater than +98%. The mean sampling bias was -17 +/- 38%. When measured in-facepiece concentrations were used to calculate a fit factor the resulting range was 44 to 4728 even though the actual fit factor was only 87. Based upon the data presented, it was hypothesized that faceseal leakage was streamlining within the respirator cavity. As a result, quantitative facepiece fit data on half-mask respirators may be biased by the large measurement error.

Acetone↗
PubMed1997

Motor unit number estimation: sample size considerations.

A computer model of the motor unit number estimation procedure was developed to evaluate the sampling error associated with estimates of the number of motor units in muscles. Two different distributions were used to model the motor unit amplitude distribution and were chosen in such a manner that they qualitatively matched the distributions observed under both normal and neurogenic conditions. As expected, the results indicated that estimation error decreases as a function of sample size. However, the relationship between these two variables was nonlinear in the sense that successive increases in sample size lead to progressively smaller decreases in estimation error. The results also indicated that the shape of the motor unit amplitude distribution plays an important role. Specifically, estimates obtained using the distribution modeling normal muscle were generally higher than the actual number of motor units in the muscle, which was not the case for the distribution modeling neurogenic muscle. In addition, the neurogenic distribution was associated with much smaller estimation error, suggesting that motor unit number estimation is well suited to the analysis of neurogenic disease processes.

Cell Count↗
PubMed1991

Protonation of interacting residues in a protein by a Monte Carlo method: application to lysozyme and the photosynthetic reaction center of Rhodobacter sphaeroides.

We used Monte Carlo methods to treat statistical problem of electrostatic interactions among many titrating amino acids and applied these methods to lysozyme and the photosynthetic reaction center of Rhodobacter sphaeroides, including all titrating sites. We computed the average protonation of residues as a function of pH from an equilibrium distribution of states generated by random sampling. Electrostatic energies were calculated from a finite difference solution to the linearized Poisson-Boltzmann equation using the coordinates from solved protein structures. For most calculations we used the Metropolis algorithm to sample protonation states; for strongly coupled sites, we substantially reduced sampling errors by using a modified algorithm that allows multiple site transitions. The Monte Carlo method agreed with calculations for a small test system, lysozyme, for which the complete partition function was calculated. We also calculated the pH dependence of the free energy change associated with electron transfer from the primary to the secondary quinone in the photosynthetic reaction center. The shape of the resulting curve agreed fairly well with experiment, but the proton uptake from which the free energy was calculated agreed only to within a factor of two with the observed values. We believe that this discrepancy resulted from errors in the individual electrostatic energy calculations rather than from errors in the Monte Carlo sampling.

Electron Transport↗
PubMed1996

Doppler grid surface scanning applications for pulmonary subsurface parenchymal perfusion assessment.

Subsurface perfusion to lung parenchyma underlying the pleura is difficult to assess in live ventilated animals. The purpose of this study was to assess applicability of a newly developed laser Doppler grid scanning imaging technology that measures perfusion of pleural subsurface lung regions in intact normal and abnormal animal lungs. Eighty-six Doppler grid perfusion measurements were performed in five New Zealand White Rabbits (3-5 kg); four with unilateral bullous lung disease, one normal control. Left upper lobe lung surface was exposed to 10 1-sec spot Nd:YAG exposures (70 W/cm2). One week following laser exposure, all rabbits underwent sequential bilateral open thoracotomy. Unaffected left lower lobes in these animals and all four lobes of a previously untreated rabbit were used as controls. Pleural subsurface perfusion measurements were recorded over a contiguous 900-pixel square surface grid using quantitative noncontact laser Doppler imaging during open thoracotomy procedures. Scans were obtained in a normal volume ventilation mode, at 30 cm of inspiratory hold airway pressure, and postinflation. A perfusion-pressure response curve was obtained in normal lung at 10-, 20-, and 30-cm static airway pressure. Post mortem measurements were used as 0 flow controls. Normal lung tissue was found to have relatively high pleural subsurface perfusion (1362 +/- 328 corrected units on a scale of 0-4095). Areas of atelectasis had decreased perfusion (659 +/- 512 U., 48.4 +/- 12.5% compared to normal lung, p < 0.02), but returned to normal levels after inflation of the lung (1253 +/- 363 U., p = 0.21 compared to normal). Pleural subsurface perfusion decreased uniformly and progressively as lung inflation pressure increased (p < 0.0001). Perfusion increased immediately to supranormal values following release of high inspiratory inflation pressure holds (1603 +/- 626 U., 117 +/- 18% compared to normal lung, p = 0.03). Bullae had markedly decreased perfusion (541 +/- 68 U.) that was not further reduced by increased inflation pressures. Noncontact laser Doppler grid perfusion imaging appears to provide a new tool for measuring pleural subsurface perfusion over a large area of lung surface in clinical experimental settings. Results are rapid, reproducible, and consistent. Sampling errors inherent in current point sampling Doppler flow techniques are reduced by the multiple contiguous measurements. We have used this technique to demonstrate inspiratory pressure-related reduction in pleural subsurface perfusion in normal lung, reversible decreased perfusion in atelectatic regions, and reduced perfusion in bullous and laser-treated lung regions.

Analysis of Variance↗