National Curriculum Levels Question

[meredeux]

Does anyone know how the individual levels for reading and writing are combined to determine a child's level for English?
e.g. If a child was a 3C for reading and a 2C for writing, what would their English be recorded as?

There's a third strand, speaking and listening, too. Is there any benefit to combining them?

My Dc's school gives them separately for the child and then combines them for reporting KS1 & 2 Sats. I was just wondering how they do that?