We have created the StandardDeviation aggregate in the CTT and our unit-tests expected to receive the exact same output as the appendix (we have replicated the datasets).

We are applying the formula as mentioned in the “OPC UA Part 13 - Aggregates 1.02 Specification.pdf” but we are not getting the output as expected.

1. Here is the Formula (see "formula.png" attachment)

2. For example take the "Historian 2" (in the spec) and Processing interval as 20,000 ms

3. Here is the part of Historian 2 data form the document “OPC UA Part 13 - Aggregates 1.02 Specification.pdf” (see "table.png" attachment)

   4. Lets consider 2nd interval 12:00:25 to 12:00:39

4. Average for the given interval is (20+25+30)/3 = 25

5. n is 3

6. Lets apply these value in the formula sqrt( ((20-25)2 + (25-25)2 + (30-25)2 ) / (3-1) )) = sqrt( ( 25 + 0 + 25 ) / 2) = sqrt (50/2) = 5

7. The expected value in the “OPC UA Part 13 - Aggregates 1.02 Specification.pdf” is 6.25

The spec says 6.25, but the CTT is saying 5. Is the CTT correct?