If you can't measure it, you can't manage it. Wise managers like to have facts and figures to support their decisions, whether they're buying banner ads or paper clips.
Online advertising is in fact an easy thing to measure and quantify. The Internet can provide a level of detail that no other medium can ever hope to. The online advertising business is a more mature one than many people realize, and the tools to measure the effectiveness of banner ad campaigns are there for those who know how to use them.

Newsletter Banners
Banner advertising is charges on a CPM rate (which stands for cost per thousand ad views). You can also get stats on how your banner has performed, i.e. how many people have clicked on your banner in the newsletter or on the web site.  
This is measured on the click-through rate (CTR) and will be one of your best measures of an ad campaign's success and campaigns perform. The ultimate result is the number of additional sales that you achieve and hence this will work out as a cost per sale figure etc.

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