Biomedical subjects

O Schabenberger

Publications and source records attributed to O Schabenberger.

5 recordsLinked to original sources

PubMed2000

Prediction from an integrated regression equation: a forestry application.

Models to depict the tapering of a tree bole abound in the literature of forest science, and such models are widely used in forestry practice. One important use is the integration of a taper equation to predict the volume of the tree bole. The statistical properties of volume prediction from an integrated taper equation have been obscure. Based on the statistical characteristics of a taper model for the bole's cross-sectional area, we derive the first two moments of the volume predictor and the prediction error. Bias from the integration is nil. The importance of a reasonable model of the error structure is demonstrated.

Biometry↗

PubMed1999

Internal pilot studies I: type I error rate of the naive t-test.

When sample size is recalculated using unblinded interim data, use of the usual t-test at the end of a study may lead to an elevated type I error rate. This paper describes a numerical quadrature investigation to calculate the true probability of rejection as a function of the time of the recalculation, the magnitude of the detectable treatment effect, and the ratio of the guessed to the true variance. We consider both 'restricted' designs, those that require final sample size at least as large as the originally calculated size, and 'unrestricted' designs, those that permit smaller final sample sizes than originally calculated. Our results indicate that the bias in the type I error rate is often negligible, especially in restricted designs. Some sets of parameters, however, induce non-trivial bias in the unrestricted design.

Bias↗

PubMed1999

Internal pilot studies II: comparison of various procedures.

The two-stage design involves sample size recalculation using an interim variance estimate. Stein proposed the design in 1945; biostatisticians recently have shown renewed interest in it. Wittes and Brittain proposed a modification aimed at greater efficiency; Gould and Shih proposed a similar procedure, but with a different interim variance estimate based on blinded data. We compare the power of Stein's original test, an idealized version of the Wittes-Brittain test, and a theoretical optimal test which can be approximated in practice. We also compare two procedures that control the conditional type I error rate given the actual final sample size: Gould and Shih's procedure and a newly proposed 'second segment' procedure. The comparison among the first three procedures indicates that the Stein test is, unexpectedly, the test of choice under the original design alternative, whereas the approximate-optimal and Wittes-Brittain procedures appear to have superior power for detecting smaller treatment differences. As between the latter two procedures, the second segment procedure is more powerful when many observations are likely to be taken after the interim resizing, whereas otherwise the Gould-Shih procedure is superior.

Analysis of Variance↗

PubMed1998

Arabidopsis CBF1 overexpression induces COR genes and enhances freezing tolerance.

Many plants, including Arabidopsis, show increased resistance to freezing after they have been exposed to low nonfreezing temperatures. This response, termed cold acclimation, is associated with the induction of COR (cold-regulated) genes mediated by the C-repeat/drought-responsive element (CRT/DRE) DNA regulatory element. Increased expression of Arabidopsis CBF1, a transcriptional activator that binds to the CRT/DRE sequence, induced COR gene expression and increased the freezing tolerance of nonacclimated Arabidopsis plants. We conclude that CBF1 is a likely regulator of the cold acclimation response, controlling the level of COR gene expression, which in turn promotes tolerance to freezing.

Acclimatization↗

PubMed1998

Comparison of gas chromatography-electron capture and enzyme-linked immunosorbent assay for deoxynivalenol in milled fractions of naturally contaminated wheat.

An enzyme-linked immunosorbent assay (ELISA) for deoxynivalenol was compared with a gas chromatography-electron capture assay to determine deoxynivalenol levels in milled fractions of wheat. The milling provided eight fractions: first, second, and third break flours; first, second, and third reduction flours; brans; and shorts. The difference between levels of deoxynivalenol quantitated by ELISA or gas chromatography did not depend significantly on the wheat samples or milled wheat fractions. For none of the fractions or samples did the differences differ significantly (P=0.05) from zero. Based on these comparative tests, ELISAs for deoxynivalenol in milled wheat fractions should provide reliable results rapidly and economically in a commercial setting.

Chromatography, Gas↗