Developer Documentation Library > Data Model > Accessing the UNICOM Intelligence Data Model > Working with the Metadata Model > Derived variables > Special elements and helper elements in metadata > Adding helper elements
 
Adding helper elements
When adding helper elements to metadata, follow these rules:
Helper elements must appear before the special element they relate to. For example, if you are creating a mean, the SumX and SumN helper elements must come before the mean element.
SumXSquared, SumX, and SumN elements must always be contiguous.
The SumUnweightedN element must always appear immediately before the standard error or sample variance element it belongs to.
If you are creating two or more special elements that use the same helper elements (mean, standard deviation, standard error and sample variance), you can create the common helper elements once, for use by all the special elements (subject to the other rules). Ensure that the helper elements precede all the special elements, and that all intermediate elements have the HasNoData property set to True.
If you are using factors on a categorical question, do not create helper elements for a mean, standard deviation, standard error or sample variance. Instead, specify factors for each category. The calculation process automatically picks up the factors and uses them in the calculation.
See also
Creating special elements in metadata
Creating special elements in metadata using a data management script
Special elements and helper elements in metadata