This is to support all derived vars (both membership derived vars, which this directly translates into generating), and any other derived vars that depend on a subset. e.g. average age of children in a household may want to exclude anyone over the age of 12 (non-children). To generate this reduction derived var, we need ALL observations, but need to filter on age when deciding whether to add an age to the mean accumulation before averaging. Also this allows us to generate a value for every row of the entity, applying a default if needed (null, 0, "", etc.), depending on the derived var plugin.
This is to support all derived vars (both membership derived vars, which this directly translates into generating), and any other derived vars that depend on a subset. e.g. average age of children in a household may want to exclude anyone over the age of 12 (non-children). To generate this reduction derived var, we need ALL observations, but need to filter on age when deciding whether to add an age to the mean accumulation before averaging. Also this allows us to generate a value for every row of the entity, applying a default if needed (null, 0, "", etc.), depending on the derived var plugin.