The board shall divide the subscribers into groups, in accordance with the nature of the business of such subscribers and the probable risk of injury therein, and they shall fix all premiums for each group in accordance with the experience thereof. Where the employes in any business are engaged in various employments in which the risk of injury is substantially different, the board may subdivide the employments into classes and shall fix the premium for each in accordance with the probable risk of injury therein.
77 P.S. § 2610