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1.
By using Monte Carlo studies, this paper compares the Welch test, the James test and the Brown-Forsythe test for comparing several means under heteroscedasticity. It appears that all the tests are quite robust with respect to departure from normality. The Brown-Forsythe test is at least as good as the James test and the Welch test; but the differences are so small that the choice is immaterial for practical purposes. 相似文献
2.
Hans-Peter Piepho 《Biometrical journal. Biometrische Zeitschrift》1995,37(2):151-160
This paper discusses tests for homogeneity of row-variances in a two-way classification. Parametric tests for this problem are shown to be highly sensitive to departures from the normality assumption. Levene's robust tests tend to be rather liberal when the number of rows exceeds the number of columns. A modification is suggested which improves the approximation for this situation, while retaining the desirable robustness property. 相似文献
3.
Abstract Extending the histogram method of Ferrenberg and Swendsen to the problem of a SAW in a bad solvent, we obtain a new expression for the free energy of such a system, which fits very properly the numerical results of our Monte Carlo simulations. The basic difference with Flory's theory lies in the reference system: instead of considering random walks (RW) we start with self-avoiding walks (SAW) for which we already proposed a model expression for the distribution of the radius of gyration. This distribution is universal for any class of homo- or heteropolymers and contains all the information concerning the excluded volume problem. For homopolymers we consider the standard case where attractive nearest neighbours interactions simulate a bad solvent. At a given radius of gyration r we compute the density of states P(r, m) for an interacting self avoiding walk (ISAW) as a function of its number of contacts m. We build a microscopic phenomenology for P(r, m) based on a factorisation procedure: P(r, m) is split into an a priori probability P(r) of finding a SAW with a given r, and a conditional probability P(m/r) of finding such a conformation with m contacts. A complete scaling is given for P(r) whereas P(m/r) is found to be well approximated by a gaussian distribution. Scaling laws for the first two cumulants of P(m/r) are exhibited from a finite-size scaling analysis. The theta-point temperature is recovered along with its dependence on the polymer length and the free energy of the globule phase is well reproduced. This approach is shown to be generalizable to heteropolymers by merely replacing m by u, the energy per monomer, and computing the density of states P(r, u) at a given r. The case of sequenced and random copolymers is examined with special attention to polyampholytes. 相似文献
4.
We present a survey of sample size formulas derived in other papers for pairwise comparisons of k treatments and for comparisons of k treatments with a control. We consider the calculation of sample sizes with preassigned per‐pair, any‐pair and all‐pairs power for tests that control either the comparisonwise or the experimentwise type I error rate. A comparison exhibits interesting similarities between the parametric, nonparametric and binomial case. 相似文献
5.
John M. Williamson Hung‐Mo Lin Tim J. Bush 《Biometrical journal. Biometrische Zeitschrift》2002,44(2):213-225
A distribution‐free two‐sample rank test is proposed for testing for differences between survival distributions in the analysis of biomedical studies in which two groups of subjects are followed over time for a particular outcome, which may recur. This method is motivated by an observational HIV (human immunodeficiency virus) study in which a group of HIV‐seropositive women and a comparable group of HIV‐seronegative women were examined every 6 months for the presence of cervical intraepithelial neoplasia (CIN), the cervical cancer precursor. Women entered the study serially and were subject to random loss to follow‐up. Only women free of CIN at study entry were followed resulting in left‐truncated survival times. If a woman is found to be CIN infected at a later examination, she is treated and then followed until CIN recurs. The two groups of women were compared at both occurrences of CIN on the basis of rank statistics. For the first occurrence of CIN, survival times since the beginning of the study (based on calendar time) are compared. For a recurrence of CIN, survival times since the first development of CIN are compared. The proposed test statistic for an overall difference between the two groups follows a chi‐square distribution with two degrees of freedom. Simulation results demonstrate the usefulness of the proposed test proposed test statistic, which reduces to the Gehan statistic if each person is followed only to the first failure and there is no serial enrollment. 相似文献