ERA-7 inning games

Question
In calculating earned run average for high school, or in this case, summer league, 7 inning games should earned run calculation be done by multiplying earned runs divided by innings pitched by 7 or 9?

Thank you.

Answer
Ken,

I know high school games are typically seven innings, but I still use 9 as the divisor for ERA.  Why?  Because that's what everyone knows.  If you start using seven, people will still try and figure out what that means as compared to what they've grown up with.

So use nine so that it keeps comparisons simple.

Hope this helps!

Brian