It seems so simple - you charge an advertiser x amount per # of impressions, insert a javascript ad tag they give you into your website et voila, you are making money serving an ad. However, there are many nuances to measuring the success of how that ad is targetted, and if you have control over variables such as exclusivity or impression spreads through a time period, things can get tricky.
Given how accesible ad serving is today, many people are avoiding Google's Adsense placements and instead looking for their own bespoke implementations of an ad server. I got thinking about how confusing the notion of ad serving is today after bouncing some concept notes over to a client and thought it may be useful to repost them here. This got penned in response to the question: "How come I can't see a particular ad in a spot live on my site even though I've reloaded the same page a few times?"
Well, there is a fine balance between how many ads share a space, the weighting of those ads individually, the periods those ads share the same space and so on. This all adds up to the fact that a banner advertisement may need many more reloads than you'd assume without working the numbers.
For example:
