where P is the fracture probability at stress o, m is the shape parameter or Weibull modulus, o 0 is the scale parameter or characteristic strength and ou is the location parameter or threshold stress. Several methods are available for the determination of the Weibull parameters, and the value of m obtained can vary according to the method employed. In spite of relatively recent contributions on the subject [1-3], it seems that no method is unanimously considered the best. This letter compares the effectiveness of several methods among the most often used and provides confidence intervals for the value of m. When analysing fracture results of brittle materials with the Weibull distribution, au is often put equal to zero (as justified, for example, in [4]) and we refer to the distribution as the two-parameter Weibull distribution. Equation 1 can be written as