Dew Shields

Does any experimental evidence suggest how much a dew shield "delays or eliminates" dew from forming on the secondary mirror of a Newtonian telescope? I have noted that most refractors have dew shields, which I suppose does something about the dew that would otherwise form on a refractor's primary lens. And I have noted that some folks construct dew shields for their finder scopes, presumably for the same reason.
I am thinking of trying a megshift dew shield for my newtonian (perhaps out of a plastic wastebasket), to deter dew from my secondary mirror, but since this will add considerable weight to the front end I am wondering if it is worth the effort.

Like Dave suggested, I think most of us wind up installing or building some kind of secondary heater if we're in a region where dew is common. The weight issue is the main thing, but also, with a large reflector, it's hard to make anything big enough that will slow dew formation down.