a) Wind statistics used for the calculation of the usability factor are normally available in ranges of speed and
direction, and the accuracy of the results obtained depends, to a large extent, on the assumed distribution of
observations within these ranges. In the absence of any sure information as to the true distribution, it is
usual to assume a uniform distribution since, in relation to the most favourable runway orientations, this
generally results in a slightly conservative figure for the usability factor.