What is a 3-Sigma rating?

Written By: Lance Piatt

What is a 3-Sigma rating?

3-Sigma ratings are used throughout the climbing and rescue industries to determine a level of accuracy in the breaking strength numbers that we report.  A 3-sigma rating is much more reliable than, say, an average strength where 50% of a sample could be below that strength.  3-sigma ratings are based on a statistical analysis of the breaking strengths of a given sample of rope. Statistically speaking a 3-sigma rating means that 99.9% of the products are stronger than the reported MBS rating. What this means is a 3-sigma rated MBS is three standard deviations below the average breaking strength.  Standard deviation helps to define the spread in the distribution of a given test sample.  It is not the mean.   Reporting 3 sigma ratings more accurately represents a rope’s strength to be at least the MBS reported.

Testing / Standards For Dynamic Ropes UIAA 101 is the standard to which all dynamic ropes are tested.  There are five areas of testing; Construction, Sheath Slippage, Static Elongation, Impact Force on first fall, and Number of falls held. UIAA breaks up dynamic ropes into three categories; Single Ropes, Twin Ropes and Half Ropes. v  Single Ropes:  One rope used independently and clipped into every piece of protection.  Mainly found in diameters from 9.0mm to 11mm. v  Twin Ropes:  Two ropes used in tandem where each rope must be clipped into every protection similarly to single ropes.  These are generally smaller diameters from 7.5mm to 8.5mm.

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