The net earnings or losses shall be apportioned among the members in accordance with the ratio which each member’s patronage during the period involved bears to total patronage by all members during the period. “Patronage” means amount of purchases, sales, business, labor, wages or other similar criteria. (1915, c. 144, s. 13; C.S., s. 5257; 1925, c. 179, s. 5; 1985, c. 542, s. 6.)