What is the test for a plaintiff to prove deliberate indifference for purposes of a failure to train?

Ordinarily, a single constitutional violation by an untrained employee is insufficient to demonstrate deliberate indifference for purposes of a failure to train. Connick v. Thompson, 563 U.S. 51, 62 (2011). Instead, a plaintiff must usually demonstrate "[a] pattern of similar constitutional violations by untrained employees." Id.