Hey guys, thank you for the discussion and help so far.

I tried to keep my doubt simple (I am not good at that) and I probably omitted some important information.

This parameter is always analyzed first against reference Standard A. If the response signal is lower than 5% of that reference, then the value must be automatically disregarded, and the parameter should be reported as “<10” (specification limit of this parameter) – end of analysis.

However, if the response signal is greater than 5% of Standard A, then it must actually be quantified. This quantification requires the use of Standard B as a reference. This consists of a different compound from that of Standard A and it is not simply a different concentration of the same Standard.

This procedure is established by guidelines and is not an in-house method, so there is no option of doing things differently, even if they sound kind of weird.

**(@supadrai) Does your signal response (assuming it is any value below "5") due to reaching the detection limit? **
There is always signal above noise level, so it is above the detection limit. I guess it can be considered that a signal response of 5% of Standard A reference to be the quantification limit of the method.

However, there is no clear relationship between this and a quantified value using Standard B. In other words, it is not clear what would exactly be the result quantified using Standard B as reference, if the signal response of the sample was exactly 5% of the Standard A. For example, this year the lowest value quantified was “5”, but last year a had some samples with this parameter quantified as “4” and I could even find a “3” in older data.

**(@Bev D) Are you creating a control chart or looking for the upper limit of the distribution? **
Kind of both. The main objective is to determine the upper control limit for this parameter, based on the recorded values, and evaluate the process capability. I have the control chart, still lacking average and control limits:

I know that the green dots are actually higher than “0”, since they signal response is always above noise level. They should be also lower than “5”, since I have several samples that needed to be quantified using Standard B as reference and gave that value. However, I also have records of “4” and even one “3” in previous years.

As a result, I know that the average would lie somewhere in between 0 and 5, but I am not sure where. I should I calculate the average in this case? Is it even correct to do so, since two distinct reference standards are used?

And how about the control limit? Is average+3sigma still applicable since my data is positively shifted?

Thanks for your help,