Confidence Factors in Risk Assessment

Hi Folks - having developed a fairly rudimentary risk matrix for the assessment of 'risk of piping failure', I need to add a confidence
factor to the formula.
Clients will be generating their own criteria for a fixed number of risks to their equipment, based on Risk, Prevention and Consequence. Each of these will have a 1 - 10 classification and a series of weightings for specific anomalous circumstances.
Not being a math wiz I'm not sure how I need to structure the confidence factor. I believe we should have a factor of CF = 1where the confidence is 100% that all criteria have been accurately captured in the Risk, Prevention and Consequence elements. However I'm not sure how to interpret or allocate factors for lower confidence. I guess it'll be something like:
1 = 100% ((Risk - Prevention)*Consequence) * CF 1) = Relative Risk figure. 2 = 95% 3 = 80% 4 = No confidence?
Any feedback would be appreciated.
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Systems exist for quantitative and semi-quantitative risk assessments of this type. You should write something that complies fully with one or more such code. The API codes are widely used for such purposes.
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