[ghshephard]

There are a lot of theories, but, the one thing that has been consistent for at least the last 60 years, is that if you have a consumer Brand of something, say "Tide Detergent" - and you stop all your brand advertising in a certain region, sales go down in that region, and when you start advertising again, sales go back up.

Price, Value, Placement, and, obviously, competition are all conflating factors as well - but the underlying response to advertising has been pretty consistent.