253.57. The units of assessment subject to the tax shall, for the purposes of the establishment of the rates, be divided into three classes.
The median class is composed of the units affected by a variation in taxable value that falls within the median level determined under section 253.55, and of the units, not referred to in the third paragraph of section 253.56, that appear on the roll coming into force and that did not appear on the preceding roll on the preceding day.
The lower class is composed of the units affected by a variation in taxable value that falls within the level containing reductions greater or increases smaller than those in the median level.
The higher class is composed of the units affected by a variation in taxable value that falls within the level containing reductions smaller or increases greater than those in the median level.
For the purposes of the third and fourth paragraphs, variation nil shall be considered to be the smallest reduction or smallest increase.