Eventhough, the long and short fibres both contribute towards the length irregularity of cotton, the short fibres are particularly responsible for increasing the waste losses, and cause unevenness and reduction in strength in the yarn spun. The relative proportions of short fibres are usually different in cottons having different mean lengths; they may even differ in two cottons having nearly the same mean fibre length, rendering one cotton more irregular than the other.It is therefore important that in addition to the fibre length of a cotton, the degree of irregularity of its length should also be known. Variability is denoted by any one of the following attributes

1. Co-efficient of variation of length (by weight or number)
2. irregularity percentage
3. Dispersion percentage and percentage of short fibres
4. Uniformity ratio

Uniformity ratio is defined as the ratio of 50% span length to 2.5% span length expressed as a percentage. Several instruments and methods are available for determination of length. Following are some

* shirley comb sorter
* Baer sorter
* A.N. Stapling apparatus
* Fibrograph

uniformity ration = (50% span length / 2.5% span length) x 100
uniformity index = (mean length / upper half mean length) x 100