What is the most effective way to market a product or service via the Internet to reach a targeted group? How would you track the activity?

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Hiring someone is probably the most expensive way to do it. Learning internet marketing yourself is probably the slowest way to do it, but the most inexpensive. Somewhere in between is pay-per-click advertising. This is the most controllable way and the quickest way to get results. Both Google and Yahoo provide you with ways to track it. The most comprehensive way to track it would be to use Google Adwords (PPC) and Google Analytics (shows you webstats by signing up and applying the code that they give you to your web pages - and also integrates with Adwords). With Adwords (and Yahoo's paid advertising) you can specify your budget per day. Depending on how competitive your product's market is, it can be very inexpensive (sometimes as little as 5 cents per click) to get targeted visitors. At 10 cents per click and a budget of 5 dollars a day, you could receive as many as 50 targeted visitors per day. Below is a source for Google's PPC advertising details.