Term can be either a score or a constant. Missing scores will be treated as zeros. All missing scores are dropped. Additional lowest scores are then dropped up to the drop count. If the number of missing scores exceeds the drop count, only the non-missing scores are averaged.
COPY creates a new score identical to an existing score.
If the original score is missing, the resulting score is also missing. If any score is missing, or if the denominator is zero, the resulting score is missing. All missing scores are dropped. Additional lowest scores are then dropped up to the drop count. The sum is based only on non-missing scores and constants.