Showing blank space when there's no suitable ad

who decides what is a suitable ad?

[londrum]

i've just been looking over my site at the ads, and a load of them are so out of whack with the page content it's getting silly. they are not even close. i cannot see even one single word on the page which could trigger the ad.

but... i know there's a shortage of ads at the moment so i'm not moaning about that. but what i'm moaning about is this: i have it set up so the ads collapse when there is nothing to show.
as far as i'm concerned, when you have a page about dogs (an example) and all they've got is ads about bananas, then they should honour your settings and make the space collapse.
but that almost never happens now, as far as I can see. they'd rather serve up some totally unrelated ad instead.

and now we are getting these new "interest based" ads, the chances of them serving up nothing is practically zero.

what i want to know is this: do you think google is serving up an ad everytime, regardless of whether they've got anything to show?

[netmeg]

No, because I have some of mine set to show an affiliate ad when AdSense isn't available for some reason, and I actually see traffic on that Affiliate link. So it must be showing some time.