Adding a CountIF to a formula that is already Countif

Morning,

I'm using this formula:

=COUNTIF('Data Sheet'!L:L, "yes")/COUNTA('Data Sheet'!L2:L225)

Which works out what percentage of Column L on the Data Sheet has the value yes.

What I now need is to check another column first, it has been suggested a COUNTIF formula. I've tried playing around with it and can't seem to get it to work, probably as I'm not sure where the all ( and , need to go

I've ordered the Excel and Formulas for dummies books so hopefully I'll learn how to do it, rather than keep posting for formulas. Until it turns up any help with this formula would be appreciated.

So, a formula that:

before it uses =COUNTIF('Data Sheet'!L:L, "yes")/COUNTA('Data Sheet'!L2:L225) it will check column b on the data sheet to see if the value in A1 on the formulas sheet is present.

Thanks for any help you can give.