This European Standard is based on the assumption that the statistical distribution of the attribute taken into consideration can be represented by one single Weibull distribution function, even where in certain cases (e.g. lifetime measurements) mixed distributions have frequently been observed. For this reason, the user of the standard has to check by a goodness of fit test whether the measured data of a sample can be represented by means of one single Weibull function. Only in this case can the hypothesis be accepted and the procedures described in this standard be applied. The user decides on this question also considering all previous relevant data and the general state of knowledge in the special field. Every extrapolation into ranges of fractiles not confirmed by measured values requires utmost care, the more so the farther the extrapolation exceeds the range of measurements.