I agree with TODave. In a truly scientific "contest," it would skew the results unfairly. But, in terms of length, I don't think it makes much, if any, difference - the gain is primarily girth because blood is trapped and erections become harder than they'd ever be without a ring. I think they also enhance the sensations - at least for the first 15 or 20 minutes, when the lack of blood supply tends to de-sensitize nerves. But even after that happens, staying rock-hard can last for quite a long time! And this is something your receiving partner can really love! For what it's worth, I prefer a ring around the base of my cock only - but many wear them around cock and balls. Your mileage may vary!