Essentially, section CPLR 203(c), commonly referred to as the relation back doctrine, "allows a claim asserted against a defendant in an amended filing to relate back to claims previously asserted against a codefendant for Statute of Limitations purposes where the two defendants are 'united in interest.'" Buran v. Coupal, 87 N.Y.2d 173,177, 661 N.E.2d 978, 638 N.Y.S.2d 405(1995). The doctrine allows plaintiff to correct a pleading error by adding either new claims or parties after the statutory limitations period has expired. Id. CPLR 203(c) states: Claim in complaint where action is commenced by filing. In an action which is commenced by filing, a claim asserted in the complaint is interposed against the defendant or a co-defendant united in interest with such defendant when the action is commenced. To benefit from the doctrine, it must be demonstrated that (1) both claims arose out of the same conduct, transaction or occurrence, (2) the new party is 'united in interest' with the original defendant, and by reason of that relationship can be charged with such notice of the institution of the action that he will not be prejudiced in maintaining his defense on the merits and (3) the new party knew or should have known that, but for an excusable mistake by plaintiff as to the identity of the proper parties, the action would have been brought against him as well. Id. at 178, quoting, Brock v. Bua, 83 A.D.2d 61, 69, 443 N.Y.S.2d 407 (2nd Dept. 1981). The word "excusable" has now been omitted from the abovementioned test. Id. The First Department has also held that it is relevant whether the failure to initially sue the new defendant was an attempt to secure some tactical advantage in the litigation. Yaniv v. Taub, 256 A.D.2d 273, 683 N.Y.S.2d 35 (1st Dept. 1998). If so, the relation back doctrine is inapplicable. Id. Plaintiff bears the burden of establishing, using proof, that the relation back doctrine applies. Teer v. Queens-Long Island Medical Group, P.C., 303 A.D.2d 488, 755 N.Y.S.2d 430 (2nd Dept. 2003); Spaulding v. Mt. Vernon Hospital, 283 A.D.2d 634, 725 N.Y.S.2d 358 (2nd Dept. 2001). Unity of interest, under the second prong of the test, will be found where there is some relationship between the defendants "giving rise to the vicarious liability of one for the conduct of the other." Vanderburg v. Brodman, 231 A.D.2d 146, 660 N.Y.S.2d 438 (1st Dept. 1997); Teer v. Queens-Long Island Medical Group, P.C., 303 A.D.2d 488, 755 N.Y.S.2d 430 (2nd Dept. 2003). The notion of control underpins the doctrine of vicarious liability Vanderburg v. Brodman, 231 A.D.2d 146, 660 N.Y.S.2d 438 (1st Dept. 1997). So that the person in a position to exercise authority or control over the wrongdoer must do so or bear the consequences. Id. For purposes of unity interest, the relevant inquiry is whether "the interest of the parties in the subject-matter is such that they the parties stand or fall together and that judgment against one will similarly affect the other." Id. at 148.