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Definitions > What is Standard Deviation of Diameter?

What is Standard Deviation of Diameter?

The Standard Deviation is a statistic, which summarise the way diameter results from individual fibres are spread around the Mean Fibre Diameter. It measures the absolute distribution of fibre diameter either side of the Mean and is expressed in microns. Sixty eight percent (68%) of fibres fall within a range one Standard Deviation either side of the Mean, and 95% of fibres fall within a range two Standard Deviations either side of the Mean.

For example, a wool with a Mean Fibre Diameter of 20 microns and a Standard Deviation of 4 microns will have 68% of fibre snippets measuring between 16mm and 24mm and, 95% of fibre snippets measuring between 12mm and 28mm.

The Standard Deviation is a statistic designed for use in describing "bell-shaped" or "normal' distributions. The distribution of fibre diameter is approximately bell-shaped or normal, and hence in this instance the SD is an appropriate statistic.

The use of SD in describing the distribution of fibre curvature is now being promoted. Fibre curvature distributions are not "bell-shaped" or "normal". Generally they are highly skewed , with the left side of the distribution fixed at zero. In such cases the SD has little statistical meaning and no practical physical meaning, although there have been attempts by some industry advisors to assign physical significance to the SD in this instance. Such significance remains to be demonstrated.