The Government Thinks It’s Legal to Access Your Emails. This Theory Explains Why – Jane Chong at The New Republic:
“[T]he “sieve theory”: the idea is that the filtration techniques the government applies to extract certain pieces of data should have implications for the legality of the initial acquisition of a much larger dataset to which the government would not otherwise be entitled. The sieve theory is kind of the converse of a much-better-understood theory of the Fourth Amendment. The “mosaic theory,” as Fourth Amendment scholar Orin Kerr has dubbed it . . . That theory posits that many otherwise-insignificant and disparate data points can be combined to yield a highly revealing—and unconstitutionally invasive—composite. Like the mosaic theory, the sieve theory is built on the idea that constitutional protections do not attach to information per se but rather hinge on how that information gets processed and used. Just as privacy proponents use the mosaic theory to argue that bits of information not individually entitled to constitutional protections might be entitled to protections when aggregated, the government is effectively using the sieve theory . . . . . to argue that a large data set comprising information to which it might normally not be entitled should be produced anyway where the government both establishes its necessity and institutes satisfactory filters to prevent its misuse. Put more simply, the mosaic theory has been used to argue the unconstitutionality of certain types of government data collection. Conversely, the sieve theory is being used to argue the lawfulness of carefully conducted government data filtration.”
More on the mosaic theory: Professor Orin Kerr‘s (GW Law School) law review article, “The Mosaic Theory of the Fourth Amendment,” 111 Mich. L. Rev. 311 (2012).