Q: In Custom Reports, what must metrics and dimensions share in order to report accurately?
A) Same view
B) Same Custom Report
C) Same index
D) Same scope
Correct Answer is D) Same scope.
Not every metric can be combined with every dimension. Each dimension and metric has a scope: user-level, session-level, or hit-level. In most cases, it only makes sense to combine dimensions and metrics that share the same scope.
Read more here: https://support.google.com/analytics/answer/1033861