Quality rule (wwQualityRule)

The quality rule can be used to specify whether values with certain characteristics are explicitly excluded from consideration by data retrieval. This parameter will override the setting of the QualityRule system parameter. Valid values are GOOD, EXTENDED, or OPTIMISTIC.

• A quality rule of GOOD means that data values with uncertain (64) OPC quality are not used in the retrieval calculations and are ignored. Values with bad QualityDetail indicate gaps in the data.

• A quality rule of EXTENDED means that data values with both good and uncertain OPC quality are used in the retrieval calculations. Values with bad QualityDetail indicate gaps in the data.

• A quality rule of OPTIMISTIC means that calculations that include some good and some NULL values do not cause the overall calculations to return NULL.

You can apply a quality rule to all retrieval modes.

The OPTIMISTIC setting for the quality rule lets you retrieve information that is possibly incomplete but may nevertheless provide better results where the calculation cycle contains data gaps. This setting calculates using the last known good value prior to the gap (if possible). The logic for determining the quality of the points returned remains unchanged. The integral retrieval mode is an exception to this where the integral is scaled up to cover gaps. For more information, see Integral retrieval.

The following figure shows a counter retrieval situation in which three of the four shown cycle boundaries are located in data gaps. Without using OPTIMISTIC, counter queries would return a NULL at all cycle boundaries because the mode needs valid good values at each end of the cycle calculate a precise difference.

If the query were to specify OPTIMISTIC, the counter mode will always return rows with numeric counter values and good quality. These rows may or may not be precise. The PercentGood column of the row returns the percentage of time in each cycle in which retrieval was able to find values stored with good quality, so if the PercentGood is anything less than 100, then the returned row may be incorrect. Quality is returned as uncertain if percent good is not 100 percent.

Now look at the counter values that are returned using OPTIMISTIC quality in the preceding illustration. The query skips the value to be returned at the first cycle boundary, because there is not enough information about the cycle prior to that boundary. At the second cycle boundary, the value 0 will be returned, because there was a gap in the data for the entire first cycle. In the second cycle, there are two points, P1 and P2. The query uses P2 as the end value of the cycle and infers a start value of the cycle from P1. At the third cycle boundary, Tc2, the query returns P2 – P1. Similarly, at the last cycle boundary, the query returns P4 – P3.

For the integral retrieval mode, the query does not summarize data for gaps because there is no way to know which value to use for the summarization. However, if the query specifies OPTIMISTIC quality, the query uses the last known good value for the summarization in the gap. As described for the counter retrieval example, the PercentGood column also expresses the quality of the calculated value in integral retrieval, so if the PercentGood is anything less than 100, then the returned row may be incorrect.