Effective reading level = native reading level/attention

That's the equation I use to guide my thinking, and that of clients, when deciding on how simple and short a piece of marketing text should be. The "equation" part is actually a bit of an overclaim, because I don't have any real numbers with which to calculate. And I suppose it should really be divided by "inattention".

But what I mean is, people aren't paying attention, particularly not to your extremely urgent marketing message. So even if they read at a post-graduate level when they are paying full attention, they read like a fifth grader when it comes to figuring out what you are saying. So you need to use the attention they have most effectively.

I love a long sentence with multiple clauses. But not when I'm trying to decide between two cell phone plans, or looking at a brochure for a product I'm not sure I'm interested in.

So, when writing a marketing piece, keep in mind the effective reading level of your audience.  If they're primed and attentive, say because they are at the point when they are comparing specs and performance for a purchase decision, you can get complicated. If you're trying to best use a tiny bit of available attention, go simple.