A slam dunk is a very successful basketball play. This is saying that the advert has had very successful ratings.
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"So what is Barack Obama up to? Well, itís an advert for his healthcare reforms. Itís already been a rating slam dunk, having been watched 25 million times in just 24 hours. "
The above is what I heard from a BBC report.
Does the underlined part make sense? If so, what does it mean?
So "rating" is correctly used here, isn't it?
I would expect "ratings."
I just thought exactly the same thing. It would be "ratings" in BrE. Mind you, in BrE, we are unlikely to use "slam dunk" - it's not unheard of but it's not yet common. Basketball is nowhere near as popular here as it in the USA.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.