The county board of equalization, when equalizing individual assessments, shall observe the following rules:
1. The valuation of each tract or lot of real property which is returned below its true and full value must be raised to the sum believed by such board to be the true and full value thereof.
2. The valuation of each tract or lot of real property which, in the opinion of the board, is returned above its true and full value must be reduced to such sum as is believed to be the true and full value thereof.