QCQA for Roads - alpha, should the LSL, and engineering limit be a mean value? Or adjust?

[Max93]

Hi,

I am doing a QC/QA analysis for road state department I'm at for my internship. But I've hit a key stumbling block.

The minimum 'engineering limit' strength is 2900 psi. The "LSL" is 3900 psi. However my understanding is that the LSL should be a minimum *mean* value for a lot - whereas in this case we begin to pay the contractor less as soon as 10% of the product falls below 3900 psi. Does this mean that for a QC/QA analysis I should account for that and make the minimum mean LSL 3900 - z (value will be -1.28155, cancelling negative) * standard deviation (for distribution, not mean), bringing the lowest mean value to above the LSL?

Or am I over-complicating it?! Should I just take the value of LSL from the spec and apply it as the critical value that less than is alpha? I am so confused and really need help!

Thank-you

 

[zelgar]

First, I would suggest you ask your fellow engineers at work this question. Part of being an intern is knowing you need help from people with more experience.

I'm guessing that LSL stands for Lower Specification Limit, meaning that if the testing results are coming in at lower than the LSL, the contractor will be penalized for continually having under spec material (i.e., over 10%). If any of the material goes below 2900 psi, that material needs to be rejected and not be used.

I'm not sure if you're trying to account for assuming a certain percentage will be less than the LSL for cost for the project with your LSL 3900 - z*SD. Considering none of the material may be lower than the LSL, you'd probably be more conservative with your budget not to assume the will be any penalty costs to the contractor. Odds are, you'll need to add some contingency costs to adjust for items that go over budget.

 

[Max93]

"First, I would suggest you ask your fellow engineers at work this question. Part of being an intern is knowing you need help from people with more experience."

I have taken this to my manager, as well as another senior Engineer in the building. I haven't got a definitive answer on this particular topic as this isn't something they have dealt with in a long time, so I thought I'd give this a crack to reach a wider audience.

I'll post again with my methodology once I have it nailed down.