Mixed sheets make students read each sign before they compute. That small step catches a surprising number of errors, especially after a week of practising one operation at a time.
They also work well as review. Once facts for all four operations have been taught, a short mixed sheet a few times a week keeps every set of facts fresh.
Questions
When should students start mixed practice?
After they are accurate with each operation on its own. Mixing too early can make it hard to tell whether an error comes from a fact a student does not know or from misreading the sign.
Are the operations evenly balanced?
Yes. Each operation gets an equal share of the problems, then the problems are shuffled.