The AdSense Algorithm

Does anyone have an idea how Google decides that an ad should be displayed?

Here is a recent post I made to one of the AdWords Groups accessed via Google:

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More options Mar 23, 11:31 pm
From: myticketgroups
Date: Sat, 24 Mar 2007 03:31:24 -0000
Local: Fri, Mar 23 2007 11:31 pm
Subject: Re: Why ad suddenly disappeared after modification
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I have no answers, just a similar experience.

I supplied a very specific phrase to the "keywords" list
for an ad I was running using Starter Edition.

I got 63 impressions based on that phrase quickly (within
a day).

I changed maximum CPC (I had none, I then made it 0.25).
(Not sure if related, but I'm mentioning it anyway).

Over the last few days, that number of impressions associated
with the phrase has not changed (still 63 after 3 more days).

I run the search of that phrase myself on Google and, not
only does MY ad not show up....NO ADS show up at all!

If no other advertiser uses that phrase, shouldn't my ad
show up?

Just trying to figure out how to properly use those keywords.

Any help would be appreciated.

Thanks!

* * * End of Post * * *

BTW: I had a click to my ad cost $2.04, bringing my total "outgo" for 2 clicks to
$2.09.

The scoreboard so far:
AdWords "Outgo": $2.09
AdSense "Income": $4.99