Abstract: To prove copyright infringement, a plaintiff must convince a jury that the defendant copied from the plaintiff’s work rather than independently creating it. To prove copying, especially in cases involving music, it’s common for plaintiffs and their experts to argue that the similarities between the parties’ creative works are so great that it is simply implausible that the defendant’s work was created without copying from the plaintiff’s work. Unfortunately, in its present form, the argument is mathematically illiterate; it assumes, without any underlying evidence, that the experts know or could reasonably estimate how likely it is that a song with similarity level x to another, earlier song was created without copying from the earlier song. Until the state of the underlying art changes, it is reasonable for experts to testify about the existence of similarities between works, but it is unsupported and unreasonable for them to testify about the likelihood that those similarities came about from copying. We don’t know that likelihood in the absence of evidence about base rates: how common it is for a song to have similarity level x with some other song in the corpus of existing songs, and how common it is for that similarity to come from copying or from independent creation (or from both songs copying a shared antecedent). Until that knowledge is available, testimony about the probability of copying should be deemed inadmissible under Federal Rule of Evidence 702.