What is 3-Sigma?

Sterling Rope uses 3-sigma testing, also known as the three-sigma rule, to evaluate and determine the minimum break strength (mbs) of our ropes.  Three-sigma is a statistical tool used to calculate probability.  Minimum break strength is very different from average break strength (abs), although, abs is used in the calculation of mbs. 

The three-sigma value is determined by calculating the standard deviation (a complex and tedious calculation on its own) of a series of five breaks.  Then multiply that value by three (hence three-sigma) and finally subtract that product from the average of the entire series.  It is 99.73% probable any additional breaks will be at or above this final value.

Sterling applies three-sigma testing for mbs to our standards because it is much more rigorous and therefore safer than using simply the average break strength.

The three-sigma process:

  1. Carryout at least 5 breaks of the item to be rated
  2. Calculate the mean
  3. Calculate the standard deviation
  4. Multiply the standard deviation by 3
  5. Subtract the product in step 4 from the mean

For those of you who are really into math and 3-sigma, there's a good explanation in this video about normal distribution: 


What is Limited Availability?

We are a US manufacturer and we keep a limited supply of select products in inventory. When your order is entered, our factory will check to see if we can ship right away or, if not, we’ll set to work to build your Sterling equipment to exacting standards. We appreciate your patience and we will contact you to confirm we’ve received your order and to let you know when it is expected to ship.

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