What is the test for determining whether a municipal authority can be held liable for failing to comply with a common law practice?

For municipal liability to be imposed, the complaint must allege sufficient facts to demonstrate that an unconstitutional custom caused the plaintiff harm. A custom is "a widespread practice that . . . is so permanent and well-settled as to constitute a custom or usage with the force of law." City of St. Louis v. Praprotnik, 485 U.S. 112, 127 (1988) (internal quotation mark omitted). Consequently, "[l]iability for improper custom may not be predicated on isolated or sporadic incidents; it must be founded upon practices of sufficient duration, frequency and consistency that the conduct has become a traditional method of carrying out that policy." Trevino v. Gates, 99 F.3d 911, 918 (9th Cir. 1996).