I’m a sucker for the predictability of a bright line rule, and Camilla Hrdy at the Written Description blog describes a possible de facto rule about the likelihood of confusion in trademark cases:
In trademark law, infringement occurs if defendant’s use of plaintiff’s trademark is likely to cause confusion as to the source of defendant’s product or as to sponsorship or affiliation. Courts across circuits often frame the question as whether an “appreciable number” of ordinarily prudent purchasers are likely to be confused. But evidence of actual confusion is not required. There is not supposed to be a magic number. Courts are supposed to assess a variety of factors, including the similarity of the marks and the markets in which they are used, along with evidence of actual confusion, if any, in order to asses whether confusion is likely, at some point, to occur.Likelihood of Confusion: Is 15% The Magic Number?
But in practice, Bernstein asserted, there is a magic number: it’s around fifteen percent. Courts will often state that a survey finding 15% or more is sufficient to support likelihood of confusion, while under 15% suggests no likelihood of confusion.
There are of course many confounding factors including whether this 15% applies to “gross confusion” (total confusion that includes noise from other factors) or “net confusion” (caused only by use of the trademark), and problems with survey evidence in general. But I’ll briefly fantasize about being asked what “likelihood of confusion” means in trademark law and answering, “15%. It’s just 15%.”