The principal causes of variability in particle size measurement, particularly in the sub-sieve range of 50 µm to sub 1 µm are summarised. The causes are illustrated with results of measurements from a series of round robins made to test reproducibility under different levels of prescription in the procedure followed. Improvements of over 50% in coefficient of variability for particle size fractions are shown to be possible when clear procedures are laid down for sampling, dispersion and handling even where different equipment constrains the exact procedure adopted. Users are encouraged to develop clear procedures based on the major factors described in the guide for each different powder encountered.