Abstract Sources of variation associated with wool fibre diameter measurement are defined and estimated for a group of 10 Romney hoggets. Other estimates for groups differing in size and age are presented which reinforce the hogget estimates and indicate limits in variation. On the basis of these variance estimates theoretical design criteria are put forward for the projection microscope technique of fibre diameter measurement. The criteria allow measurement of the fibre diameter of individual or grouped sheep with a precision commensurate with the purpose for which the measurements are taken. The practicality of the criteria for routine testing is now being assessed.
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