N.Y. Comp. Codes R. & Regs. tit. 12 § 250.13
The term showing of interest, as used in this Chapter, shall mean a demonstration of support for the filing of a petition for certification, a motion to intervene, or for certification without an election. A showing of interest shall consist of evidence of current membership in a labor organization, including dues deduction authorizations, original designation cards, or petitions on a form prescribed by the board, all of which were signed and dated within one year of submission. A showing of interest for the filing of a petition or a motion to intervene must consist of at least thirty (30) percent of employees in an allegedly appropriate negotiating unit or a negotiating unit determined to be appropriate. Any showing of interest must be accompanied by a declaration of authenticity as set forth in section 251.4 of this Chapter. A filing of dues deduction cards or other evidence of support sufficient to demonstrate majority support in a unit alleged to be appropriate may also be used to determine whether a labor organization is entitled to certification without an election pursuant to section 251.4(b) of this Chapter.
N.Y. Comp. Codes R. & Regs. Tit. 12 § 250.13