What is the test for reasonable suspicion of criminal activity?

In sum, an officer having a reasonable suspicion, based on articulable, objective facts, that criminal activity is afoot may make an investigatory stop that is reasonable in both its duration and its intrusiveness into the individual's legitimate privacy expectations. In balancing the government's law enforcement interests against the individual's interests it is generally true that the more intrusive the stop, the stronger the justification need be, see Michigan v. Summers, --- U.S. ----, 101 S.Ct. 2587, 69 L.Ed.2d 340 (1981); United States v. Vasquez, supra, 638 F.2d at 520, and that "if probable cause is lacking, the intrusion must be no greater than the circumstance require," id.