Skip to Main Content

Substantial Similarity

Illustrates the "substantial similarity" doctrine from U.S. copyright law, using a set of case summaries.

Computer Associates International, Inc. v. Altai Inc., 982 F.2d 693 (2d Cir. 1992).


The defendant, Altai, published a computer program, OSCAR 3.4, after hiring someone who had worked for the plaintiff, Computer Associates. About 30% of the source code of OSCAR 3.4 came from Computer Associates’s ADAPTER program. Upon learning of the infringement, Altai initiated a rewrite of OSCAR 3.4 and published OSCAR 3.5. It took eight programmers about six months to write the new program. None of the programmers had worked on OSCAR 3.4. OSCAR 3.5 did not share any source code with ADAPTER.

The district court found infringement as to OSCAR 3.4 but concluded that OSCAR 3.5 was not substantially similar to ADAPTER. Computer Associates appealed the judgment as to OSCAR 3.5.

Procedural Notes

The Second Circuit permitted the use of expert testimony, even though it noted that the “general rule in this circuit has been to limit the use of expert opinion in determining whether works at issue are substantially similar.” The court noted “that computer programs are likely to be somewhat impenetrable by lay observers—whether they be judges or juries—and, thus, seem to fall outside the category of works contemplated by those who engineered the Arnstein test.” The court concluded that it should be in “the discretion of the district court to decide to what extent, if any, expert opinion, regarding the highly technical nature of computer programs, is warranted in a given case.”

Court’s Analysis

The Second Circuit upheld the district court’s decision on substantial similarity.

The court first noted that the literal elements of OSCAR 3.5 were not substantially similar to those of ADAPTER. For the non-literal elements, Computer Associates argued “that, despite Altai’s rewrite of the OSCAR code, the resulting program remained substantially similar to the structure of its ADAPTER program” and that it was “also substantially similar to ADAPTER with respect to the list of services that both ADAPTER and OSCAR obtain from their respective operating systems.”

The court then considered “whether and to what extent these elements of computer programs are protected by copyright law.” Finding first that Congress intended computer programs to be protected as literary works, the court then considered the idea-expression dichotomy. Analogizing from Baker v. Selden, 101 U.S. 99 (1879), the court concluded that “those elements of a computer program that are necessarily incidental to its function are similarly unprotectable.”

Saying it was drawing on “such familiar copyright doctrines as merger, scènes à faire, and public domain,” the court proposed the “Abstraction-Filtration-Comparison” for considering substantial similarity between two computer programs:

[A] court would first break down the allegedly infringed program into its constituent structural parts. Then, by examining each of these parts for such things as incorporated ideas, expression that is necessarily incidental to those ideas, and elements that are taken from the public domain, a court would then be able to sift out all non-protectable material. Left with a kernel, or possibly kernels, of creative expression after following this process of elimination, the court's last step would be to compare this material with the structure of an allegedly infringing program. The result of this comparison will determine whether the protectable elements of the programs at issue are substantially similar so as to warrant a finding of infringement.

The Second Circuit then reviewed the district court’s opinion using this test and found that it had reached the correct conclusion. For example, “the district court . . . found that the overlap exhibited between the list of services required for both ADAPTER and OSCAR 3.5 was . . . dictated by the nature of other programs with which it was designed to interact and, thus, is not protected by copyright.” In addition, the district court had characterized the programs’ organizational charts as “simple and obvious to anyone exposed to the operation of the programs,” so it gave no weight to their similarities. The Second Circuit found this was appropriate as an application of the scènes à faire doctrine.