They may rely on expert or lay observation or both and may subjectively judge the feel of a work or critically analyze its elements.
[1][3] Under the doctrine of substantial similarity, a work can be found to infringe copyright even if the wording of text has been changed or visual or audible elements are altered.
Some courts use "striking" or "probative" instead of "substantial" to describe the level of similarity needed in the first context to avoid confusion.
[1][page needed] The second meaning, which Justice Jon O. Newman referred to in 1997 as the more proper use, defines "the threshold for determining that the degree of similarity suffices to demonstrate actionable infringement" exists, "after the fact of copying has been established.
Among these are: Generally, copying cannot be proven without some evidence of access; however, in the seminal case on striking similarity, Arnstein v. Porter, the Second Circuit stated that even absent a finding of access, copying can be established when the similarities between two works are "so striking as to preclude the possibility that the plaintiff and defendant independently arrived at the same result.
It is found when similarity between the copyrightable elements of two works rises above the de minimis exception, reaching a threshold that is "substantial" both qualitatively and quantitatively.
[3] The substantial similarity standard is used for all kinds of copyrighted subject matter: books, photographs, plays, music, software, etc.
[1][page needed] In situations where "reasonable minds could not differ" in the opinion that substantial similarity of expression does not exist, a court may make summary judgment for the defendant, closing a case without finding infringement.
[16] Since "substantial similarity" can require careful evaluation, however, infringement cases usually lead to full inquiry with appropriate tests developed by the courts.
These may rely one or both of expert or lay observation and may subjectively judge the feel of a work or critically analyze its elements.
The differences between the two were defined in 1977 by United States federal judge James Marshall Carter in Sid & Marty Krofft Television Productions, Inc. v. McDonald's Corp.:[25] [The extrinsic test] is extrinsic because it depends not on the responses of the trier of fact, but on specific criteria that can be listed and analyzed.
The test to be applied in determining whether there is substantial similarity in expressions shall be labeled an intrinsic one depending on the response of the ordinary reasonable person.
[27] In Brown Bag Software v. Symantec Corp, the Ninth Circuit expanded the extrinsic test to include the analysis expression as well as ideas.
[19][29] The test, which was devised by the U.S. Court of Appeals for the Second Circuit for Computer Associates International, Inc. v. Altai, Inc., compares the elements of software at increasing levels of abstraction, from machine instructions to program function, excluding those elements not copyrightable, such as those approaches dictated by efficiency or the fundamental operation of computers, to evaluate similarity.
The inverse ratio rule holds that the more an alleged infringer had access to a work, the lower the threshold for establishing substantial similarity.
Two recent cases signaled changes in the Ninth's attitude towards the inverse ratio rule: the suit of Marvin Gaye's estate over "Blurred Lines" by Robin Thicke, and the suit brought by a trust for the band Spirit over Led Zeppelin's "Stairway to Heaven".