Group contracts, covering at least 50 employees or members, may provide for the adjustment of the rate of premium at the end of the first year or any subsequent year of insurance thereunder based on the experience thereunder both past and contemplated. No medical service corporation shall use any form of experience rating plan until it shall have filed with the commissioner the formulas to be used and the classes of groups to which they are to apply. See definition section N.J. Rev. Stat. § 17:48A-1.